Vintage Synth Repair

Hi everyone ~ quick question:

I've got a Micromoog that's in pretty good shape but I notice that one or two keys are audibly more distorted that the rest. I can only describe it as a type of harmonic distortion. With the synth set to "normal and reasonably clean" settings (no osc mod or full / tone filter settings on) almost all the notes are fine. The two or three specific ones have got that distortion though. Couple of caps going or something? As always, thanks.