This may sound silly but I had what sounds like the exact problem.
If you are running wide wheels (I've got 18x10 XD9's) the lip of the rim can get awfully close to one of the arms underneath (toe arm from memory) and I found that the tyre company I used to fit and balance my tyres had put wheel weights on the outer lip, this causes a knock every know and then at low speeds going up and down driveways/speed bumps and even turning... The noise also got more frequent with more weight in the car.
I triple checked everything and was just laying under the car thinking WTF then I spotted how shiny the lead weight was and the witness marks around the lip of the rim of where it had been sliding around every time it clipped the arm.
Popped it off never had a drama again
