Hissing sound, trans related?

[wired1000]

Hi all,

It's gotten pretty cold out, and I've started to hear a funny noise... A hissing sound, like compressed air escaping. Seems to be coming from the front of the vehicle.

It seems to happen when the engine is at higher revs, actually it seems like it happens mostly at times when the trans should be shifting, so like higher revs but not even-higher revs (hard acceleration makes it go away).

I can't reproduce it all the time, but it's been happening more and more. As this is my daily driver, I need to figure out what's up and make it go away. The vehicle seems to be operating just fine, only making this funny sound which it didn't used to do.

I saw another thread which mentioned a hissing sound... a guy's transmission wasn't shifting at all. I was planning to start by replacing all the vacuum lines.

Any other ideas?

Last bit of info which may or may not be related. A few weeks ago I tightened up my flex plate bolts, and during that operation I had loosened one of the trans oil cooler lines. I forgot to tighten it, and ended up leaking a lot of fluid... so when I noticed that, I added 1.5 quarts and tightened the line, which seems to be holding. I only mention this because.... ??? maybe it's related?


Appreciate your help in advance!!!

(Btw I've got an M1009 which doesn't seem to be modified other than having a NP205... engine/trans is stock)

 

[CUCVFAN]

Vacuum valve/lines on the IP?

 

[wired1000]

Vacuum valve/lines on the IP?

Yes, my first course of action is replacing the vacuum lines.

 

[wired1000]

Vacuum valve/lines on the IP?

Does the vacuum system really produce so much vacuum that a leak could make a hissing sound would be audible inside the truck while it's driving ?? (truck is very noisy)

 

[ken]

Are you hearing this sound at about 50 to 60 MPH? With a light throttle? Is the tranny shifting normally? I have heard a similar sound with many of my GM vehicles since i was a teenager. I've heard it with TH350 trannys and TH400's i have owned. But never had a problem. But more throttle or a loaded vehicle makes it go away. My 86 M1028 does the same thing. But runs great. If i put the M101 behind it or load the bed it stops.

 

[wired1000]

The sound happens related to engine speed, not vehicle speed... so it happens at lower speeds as well... like I said it seems to be when the trans is getting ready to shift (1st to 2nd, 2nd to 3rd, and also in third at higher engine speeds) I assume my trans is still the stock TH400. It's a 3-speed anyway.

 

[CUCVFAN]

Does the vacuum system really produce so much vacuum that a leak could make a hissing sound would be audible inside the truck while it's driving ?? (truck is very noisy)

I was thinking this after I re-read your post. If you can hear it over the engine at highway speeds, it is not a vacuum leak.

Alternator bearings? Take the belts off and sping them by hand to see how noisy they are.

 

[Barrman]

My 6.2 diesel USAF Suburban started doing the exact same sound yesterday leaving work. The 1-2 shift usually happens about 8-15 mph. It didn't yesterday. The engine started to wind out, I heard the hissing and then at 22 mph it went to second and at 28 mph it went to third.

I am very confident it will be a bad vacuum line between the valve on the side of the IP and the transmission modulator. More than likely it will be in one of the curves the vacuum lines make. I just didn't have time to mess with it last night or this morning. Did it every time I had to go through the gears though.

Here is what the -20 says:

50. NOISY (WHINES OR “KLUNKS”)​

​

​

​

​

Step 1. Check transmission fluid indicator for proper reading. (See TM 9-2320-289-10)
Step 2. Check transmission mounting for looseness or damage.
Tighten any loose retaining screws to 80 lb.-ft. (108 N•m). Notify your
supervisor if there is any damage.​

51. WILL NOT SHIFT PROPERLY​

Step 1. Check transmission fluid indicator for proper reading. (See TM 9-2320-289-10)
Step 2. Inspect vacuum hoses, vacuum pump, and vacuum regulator valve for improper
connection, leakage, or damage.
Connect or replace vacuum hoses and vacuum pump as required. (See
paragraphs 3-21 and 3-7) Adjust vacuum regulator valve. (See paragraph
3-21.1)
Step 3. Inspect vacuum modulator (right side of transmission near base of transmission fill
tube) for leakage and damage.
Tighten mounting bolt to 20 Ib.-ft. (27 N•m). Notify your supervisor if damaged​

or still leaking.

 

[CUCVFAN]

Let me clarify what I said. If the leak is audible over the engine noise, it would be a BIG leak, which, to me, would mean the shifting would be noticeably affected, which did not sound like the case here?

 

[idM1028]

Alternator bearings? Take the belts off and sping them by hand to see how noisy they are.

I doubt it would be the alternator bearings. I had my GEN2 rebuilt earlier this year and the front bearing was pretty loud. It was your typical noisy/bad bearing sound: gritty, scratchy, metal-on-metal, but hissing it was not.

 

[wired1000]

Replaced the vacuum lines for the transmission. They were old as **** and basically cracked off in my hands. Glad that's taken care of. The sound, however is still there. It's very weird... happens only at like 80% of engine's redline. Maybe I'm just finding the harmonic frequency of some obscure part, and it only has the space to vibrate and make that sound at cold temps?

I did not have time to check the alternator bearings, but I agree with idM1028... the bearings would probably make noise at most if not all engine speeds... right?

I did check my trans fluid level, and it seems like I overfilled it by an inch or so on the dipstick... could that be related?

 

[wired1000]

I'll have to check through the lines one more time... I did not look at the hard lines or the ones down at the trans itself (if there are any... I feel like I saw one down there, but I haven't looked lately and can't remember)

Can one of those other vacuum components (pump, modulator, etc) make that sound when they're on their way out?

 

[idM1028]

... the bearings would probably make noise at most if not all engine speeds... right?

Eh, about the only time I could hear it over the engine was at idle. My dad and I did stick an automotive stethoscope (no, I'm not making this up. They really make them and they really are an amazing tool) on it one night and you could definitely tell it was noisy. I grabbed the alt out a month or two later to have it tested. Thought it was done (bad alt in a CUCV? Noooo... ) Tested fine, but the guy at the shop was like "Yeah, your front bearings kinda noisy..."

 

[Barrman]

Glad you got your shifting fixed. I plugged one of the vacuum lines all the way back in on mine and all is well.

I have been thinking about that noise though and think I found it. Air filter inlet. The Bomber had a plastic noise baffle on the intake when I got it. No outside air inlet. While replacing all the a/c parts under the hood this summer, I took it off and never bothered to put it back on. I fixed my shifting and started playing with the throttle during shifting to make it not shift right and make the noise. I could get the shift, but not the noise everytime.

I put the resonator on there and the noise didn't happen at all. I took it back off because with no gauges in these stupid trucks. I figure it can be my pre yellow line alert.

 

[wired1000]

Barrman:

Well, my shifting was never broken. It still shifts the same as it always has... just makes this hissing sound right before a shift (about 50% of the time). It could be that I have just not located the source of the problem.

About this resonator / baffle on the air filter inlet... can you describe in more detail? I'm not sure I understand what you mean. Maybe take a picture or two?


Thanks

 

[wired1000]

@ idM1028: I have an automotive stethoscope. It was on sale at Harbor Freight Tools for like $1.98 or something.

 

[Barrman]

CUCV trucks all had the round air filter with a single oblong shaped air opening over the passenger side valve cover. Basically, the same air filter housing the civilian trucks used. GM used plastic ducting/tubing and such to try and quiet down the air noise heard inside the truck. I know they have several different versions depending on the year of the truck.

The 1984 C20 6.2 Suburban had a plastic thing about 1 foot long attached to the air cleaner housing. I don't have any pictures here at work, but I will try and remember to take one.

 

[CROM]

I was having a wierd whinning/hissing sound for a bit, as well. The noise happened all the way up to when the truck would shift, then quit right before the actual gear shift only to pop up again before the next shift. I was also able to hear it a high speeds.

What fixed it was a trans filter/fluid change. When's the last time you dropped the pan? The filter and gasket come in a kit for about $14. New dexron varies.

 

[Barrman]

I was going to suggest that as well. When I first heard the sound and had a delayed shift. I checked the fluid level. It was fine, but there was a fine coating of the silver slimy stuff on the stick. Less than 2000 miles on the filter and fluid, but before that, who knows? I got it off the GL lot so it could have been decades since the Air Force did maintenance on the transmission.

Dropped the pan and put in new fluid and filter yesterday. The magnet didn 't have much on it. However, there were little stalagtite formations of the slimy metal off the filter. I did it at work so there are no pictures. I had to pull a loaded trailer leaving work with hit yesterday and it shifted great. I think my missed shift was a combination of the vacuum leak and the filter being clogged up with 25 years worth of crud finally getting moved out of the transmission.