The fat stainless 6 blade fan in my 32 has a low pitched whine to it when running 80 or so. (3300 rpm.)

One test that may help is to drive at the problem speed, put the trans in neutral and let off the gas - at the same time - so that all you're hearing is the drive train.

Noise goes away points to the engine.
Noise stays, drivetrain probs.