Toyota Repair: Rear end whine, rear drum brakes, drum brakes


Question
Hi Ted, I have a '93 Toyota Previa. 2wd,rwd,manual tranny, non super-charged. no ABS and rear drum brakes. Recently I've developed a whine coming from the rear of the car. It starts at about 35 mph and increases in voulume as I speed up. It goes away as soon as I depress the clutch pedal. It seams quiter when i'm going uphill or when there is weight in the rear of the car.  Also, it gets louder as I drive more. It's a little bit quiter when I go downhill.Turns make no difference. Any ideas? What is a good way to identify source.  

Answer
I need to ask a few questions before I can give an answer:
Is the noise the same in all the gears?
Does the noise go away in fourth gear?
With the car stopped,engine running, is there a noise in neutral and does it go away if the clutch is engaged?
I'm trying to determine if it's the transmission or the differential so please answer these questions as accurately as you can.