I have a '99 RX300 AWD with about 146k miles on it. For the past few thousand miles, it has been making a whining noise at speed when the accelerator is depressed. Once I take my foot off, it goes away.

http://www.youtube.com/watch?v=0CK8zAAw4AA

This is not my car, but it sounds very similar, perhaps not quite as loud though.

 

Could be your tranmission....just a guess...

 

Mine made a whining noise when it was coasting when the transmission was going. It would occur around 35 mph. However, before concluding it was the transmission they did change the differential and a number of other items. I'm sorry I don't remember all the details as it was just before the original warranty was up, and that was some time ago.

 

sounds like the diff. or the rear to me.some times when the gears wear,and the backlash is not to spec any more,this noise can happen.