My 84 300td makes a humming sound, more noticeable in the freeway speeds. It makes two pitches depending on if I press the accelerator or release it. Is it the drive shaft, transmission, wheel bearings or something else. Thanks for all you guys help, this is a wonderful forum.

i'm going to speculate that this is not enough information to narrow in on the problem. maybe you can have someone experienced drive the car, or look at it while running on a dynamometer.

It's hard to say without seeing/hearing in person, but that sounds like a pinion bearing, or possibly a carrier bearing, on its way out.