Front end thumping noise at >30mph

I have a 2005 Prius which recently started making a soft and rhythmic thumping noise from the front end. This starts when driving at around 35mph, and changes with the speed of the car. It feels like it is coming from right under my feet, whether on the driver's side or passenger's side. I do not hear it any better with the window open.

There is also a clunking noise when I go in reverse and then step hard on the brakes, though that has existed for at least 3 years.

The wheels were inspected, balanced, and rotated by the shop that installed them originally two years ago. I replaced the brake pads, the suspension and stearing has been checked by two mechanics (Monro, and an independent shop). Neither shop found any problems. The driver's side axle was replaced because the boot was ripped and the grease had come out (I think the replacement was overkill, but whatever). The front shocks and driver's side wheel bearings were replaced were replaced two years (20k) ago at a shop. None of these checks or fixes gets rid of the sound (or my concern).

The sound started a month after driving down a steep hill, where something when *clunk* in the driver's side wheel area, and seemed to catch between the brake pads. I could not find what caused the noise.

I was told it may be a transmission or gearbox or something like that, but they were not transmission experts.

Any thoughts?

 

220,000 miles.

And I should add that there is no vehicle shaking, and the steering/alignment seems ok, even without having had a recent alignment.

 

The wheel bearings and struts were replaced two years ago, and the axle was yesterday . . . the thumping has been going on about a month now. Mechanic heard the noise.

I vaguely remember the last mechanic saying something about hearing a noise (maybe it was in the gearbox area? I forget. . .) when he turned the wheel.