M998 drivetrain issues please help !

[Mikei998]

So my m998 started acting up recently . It started with a new vibration and when I let off the gas to come to a stop I get a pounding sound . Doesn’t act up right away . Needs a couple miles then is really bad . Let off the gas and the pounding sound starts until you come to a stop . Any ideas ?

 

[DREDnot]

My first thought with drivetrain noise that is affected by letting off the throttle would be U-joints/center bearing.
Mine had a ringing squeak that matched driveshaft speed under a medium acceleration. Greased all the joints. They seemed in good condition but when I replaced both front and rear shaft assemblies with NOS spicer made surplus shafts, the noise went away and it runs smoother now.

 

[Mogman]

I agree it could be drive shaft related, but of course you also have 4 half shafts, and 2 differentials to consider, most folks can tell the difference between drive shaft speed vibration and wheel speed vibration, of course the half shafts are twisting at twice the speed of the wheels.
Chock the wheels, release the park brake and crawl under there and push-pull-twist on all the shafts (cold of course) and see if you can find anything with slack, look for rust colored dust around the u-joints and support bearing, check the flange bolts where the half shafts bolt to the differentials.

This does sound like a "worry" noise and you should get a handle on it as a loose shaft (as in get loose) can cause a ton of damage.

On the other hand having to warm up sounds like possibly a diff. Of course the diffs and gear hubs have been serviced yes? make sure there is no excessive play on any of the three differential shafts on both diffs.