Transmission roaring?

We have an `02 Rendezvous and recently it sounds like the transmission is making a soft roaring noise when it shifts into 4th gear. I'm pretty sure it's not the lock-up clutch because if I touch the brake pedal while it's in 4th you can see the rpm's jump a bit but the noise stays the same. There are 66,000 miles on the transmission and the fluid was changed at 50,000 miles. Anyone here had this problem?

I have a very similar problem. The intermittent Growling noise occurs when the car is at steady rate of speed and the transmission drops down into over-drive (tachometer reads 1100 to 1400 RPMs). The noise is the loudest or most obvious when the engine is warmed up and the car is traveling at 30 to 40 MPH.
Did you ever resolve it?