My 200,000 miles 1999 rx300 awd has a whining noise from under the chassis around 30-40mph. It is not related to engine rev but the noise level changes with vehicle speed. The noise disappears after driving for 10 minutes or so. I shift the gear to neutral at the speed. The noise remains without changing its pitch. Could it just be the sign of a dying transmission? I've done the following but of no help.

1. Changed out serpentine belt and power steering belt.
2. Changed out transfer case fluid.
3. Replaced transmission filter and refilled fluid with Toyota Type VI. There is no metal shavings in the pan.
4. Timing belt, tensioner & pulley, idler pulley, and water pump were replaced at 180,000 miles with Aisin kit.
5. Two drive shaft supports were replaced at 150,000 miles with Toyota parts.
6. No vibration or wobble on all four wheels.

My last guess will be one of the wheel bearings. By opening up each windows in turns, I find the noise level remains constant. I'm puzzled.

 

Get a tire tread depth gage and check your tires. They could be the cause of your whining noise.

 

When I had my RX300, it would whine when it was cold outside. I would get a noise from the transfer case around the 20-30 mph area and as the speed increased, it would diminish. Also, after driving the car for a while and allowing the transfer case/transmission to warm up, the noise would dissipate as well.
Could be you have a similar inherent noise??

 

That's exactly my problem. It was above 50s in the past couple of days. The noise pitch changed and would dissipate quicker. I've just had my tires re-balanced and rotated. The shop couldn't find any fault with the tires. They are Firestone Destination LE, less than 10,000 miles. One other thing I suspect is the tire compound since the noise has very much to do with the climate.

 

Whining is whining. Something spins very fast. Rent car lift, gid'er up and run engine with gear engaged. Then you can maybe spot noise origin from below. Cars are large metal boxes, sound transfers easily and is very hard to pin point. My next guess will be hub bearing is not liking all the mileage anymore. Also, water pumps are known to do cold whine that goes away as it all warms up.