Very hard to diagnose these things without hearing and feeling them. You mention hearing it when going straight. Does it change as you turn left or right at those speeds. (need to test on long sweeping turns or clover leaf ramp) Do you hear a clunk when fist letting out the clutch from a stop? How about if you cruise in 2nd gear and get lightly on then off the throttle? Bad joints may tend to clunk under those conditions. What seems odd is that it makes noise only at two specific speeds. But more testing and observing under various conditions is always a good start. When you jack it up, grab the tire on either side and rotate sharply one direction then the other. Does is clunk? Compare to the other side. Unfortunately most of my experience is on cars with old-fashioned U-joints in the half-shafts so I’m not sure how these act when they fail. Larry or DanO, you out there?
Also think about wheel bearings, which can make considerable noise, although it would still be odd that the sound is only at a couple speeds.
