Have an unususal noise from the drivetrain. It starts at about 45 MPH and gets louder (but with no change in frequency) as MPH increases. It sounds like a dragging brake or a vacuum leak, but it is strictly MPH related (not RPM). The faster one goes, the louder it gets, and in coast or drive, and engine off or on; it stays the same.

The Tech used an acoustic noise finder and it provided no clues while driving. By putting the car on a lift, and running it in gear, the sound seems to come from THE MIDDLE OF THE DRIVESHAFT, and it is LOUD! The U-joints were inspected, and I just had the differential pinion bearing replaced due to it's failure, and a new axle and all other differential bearings were replaced at that time, but the noise started BEFORE this work was done.

There is some faint bearing noise at the tranny tailshaft, but a pan inspection showed normal wear debris and no bearing or bushing residue. What about the transmission oil pump? There are no codes being thrown, nor any shifting issues?

The car has 86K miles and is stock and has not abused. Tires have been rotated and pressures checked, and no evedence of dragging brakes or other related problems.

What about the propeller shaft? Could a balance weight have fallen off? Would rotating the shaft 180 degrees assist in diagnosis? Can the GM EVA tool pick up acoustic noise, as there is no apparent vibration?

What about this torsional rattle that is mentioned in the shop manual? Google does not offer much help on this.

Any other ideas?

BTW, this is an update to a "dead thread"
http://web.camaross.com/forums/showt...83#post4009983