I had one on an RX Pro that just about put me in the looney bin. I would suggest that if you haven't already, make double sure the pedal is tight, as well as the crank arm, on both sides....
My problem, which I chased for 2 weeks, was the damn seat. I finally noticed that when I shifted my (rather significant) weight just right, the bike made that sound I was chasing. A few minutes later it dawned on me that the seat was moving in sync with my leg movements. That was the moment I fell back in love with my new bike.... -Al