I occasionally encounter the same problem with my Mono/Poly. I've discovered it is nothing wrong with the Mono/Poly but rather is down to the fact that the synth picks up ground hum from other instruments (SOMETIMES).

It all depends how many bits of gear I've got setup and how they're all wired etc

Anyhow, it can be eliminated with a simple and cheap audio isolator from Orchid Electronics (or similar)

Maybe your problem is different, but like I say, mine does it from time to time - the hum comes from the channel I'm feeding the Mono/Poly into on my desk, but is not actually generated by the synth itself - more so by the spaghetti junction of cables running around my studio :)