M923 weird air noise?

[serial14]

I must bow down to the great knowledge of the SS crowd on this problem. Hopefully the collective wisdom can give me a hint.

I working through the wonderful joys of trying to register my M923A1. Since I don't have things squared away yet I don't drive it too much, just a little around the 'hood each weekend to appease my desire to drive it. So in short... really easy driving. When I first got the truck everything worked perfectly, no leaks, no weird noises, etc. A couple days ago it started making a weird noise and I can't find anything out of place. And since I haven't been beating on the truck, I don't think I caused the problem.

Here is what I know
- It only happens when I'm moving. Stopped it makes no noise.
- Braking, not breaking, etc doesn't make a difference
- It sounds like an air leak.. and like a hose flopping around but I can't find any broken hoses.
- It sounds more related to engine speed rather than vehicle speed.
- My air gauges work and don't indicate that i"m loosing air
- My compressor cycles on and off as expected from normal brake operation. So I don't think that I'm always pumping to keep up with a leak.
- When I'm in the drivers seat, it sounds like the problem is towards the rear of the truck.

Any ideas, suggestions, or solutions?

Thanks

 

[doghead]

Look over your air operated fan clutch. There have been a few people with leaks lately.

 

[M35A2-AZ]

My lift side windshield wiper was leaking a little and I could hear it off and on.
I had to get my wife in the truck to find it.