I'm just wildly guessing, which probably is what I do most of the time...
One leg slightly shorter or cleat position off slightly on one side? Maybe the saddle height is a tad too high. I'd try lowering it maybe 4 to 6 mm at a time (1/8th to 1/4 inch) and see if that helps.
Measure to some point on it so you know where it started out. If you don't think anything better is happening by the time you made several adjustments, then probably that's the wrong thing to be doing. Put it back where it was.