If a few shops have looked at it, it is very unlikely to be any of the obvious things mentioned, like tire rub, etc. I assume it happens with the wheels straight, as it is pretty hard to get a full compression when sharply turned (and very hard on parts). Noises under suspension compression can be tough to find because the spring keeps you from actually being able to duplicate it on the hoist. Sometimes the only way to find it is to disassemble the parts to look for damage, or remove the spring so you can move the suspension with it on the rack.
Most likely it will turn out to be something like the a-arm pivots getting metal to metal at some point, a shock with a bad spot, loose wheel bearings allowing the rotor to hit the shield or something else.
Good luck finding it, and when you do, make sure you do the other side at the same time, as it will probably be going also.
