My older sister has a 2005 Buick Rainier suv (same as GMC Envoy/Oldsmobile Bravada/etc), with the 5.3 LS V8 and full-time AWD. She says lately it's been making a "strange noise" while driving, louder when she lets off the gas. I went and drove it around for a while with my brother while we both listened to it, and near as we can make out, it sounds like gears that are not meshing smoothly, as if they're worn or low on lubrication. We hear it mostly in 3rd gear, around 35-50 mph, louder when I let off the gas. I checked all the fluids I could (oil, ATF, power steering), the transfer case has no dipstick but may have fill and drain plugs, couldn't crawl under to check. Vehicle has 135K miles on it, some towing.
Noise is not engine speed related. No strange shuddering/jerking/wobbling. All 4 tires same type, size, air pressure. Don't hear it when driving circles in parking lots, either direction.
It doesn't sound or feel like bad U-joints, but I suppose it's possible, considering the sound is louder under decel than accel - but it sounds fine in 1st gear, normal in 2nd, can hear it in 3rd, and a little in 4th.
Are the fulltime transfer cases in these vehicles chain or gear drive? Do they have a history of known failures or problems?
Only thing I could think for her to do was to have the fluid in the transfer case and transmission drained and refilled with the proper type, in case they were low or contaminated.
