Just to follow up; I just got the OT back and gave it a thorough inspection. It sounds like the springs jiggling a bit inside the buttons. The sound is not concentrated anywhere in particular, you hear it all over the front panel.
I don’t think it’s anything to worry about, I ran the self test in the early startup menu, checked all the knobs, buttons, crossfader, etc. Then proceded to jam on it for like an hour and a half and wonder what made me sell it in the first place! (oh yeah, the lust for more gear as usual.)
Anyway I grabbed a Microbrute and a OP-1 over the past few weeks and these both seem like fun OT companions so I have a feeling that it might stick around for a while, at least till the next lusty piece of gear comes out.