Thanks Ben. I'm beginning to think you may be right. It only happens on a few patches, like I said. Also, I mostly play with headphones on, so I'm probably hearing things that someone listening through monitors might not.
Unfortunately your parameter change instructions didn't make it go away either, as did not changing envelope attack or decay assignments in the mod matrix. It doesn't really matter though... as long as it seems there's nothing wrong with the unit, which my intuition tells me is true. Actually, I haven't had much chance to play around with it, but I'm beginning to think that changing the oscilator waves and or changing the tuning relations between them might do it. It seemed to work this morning. Anyway, thanks for your help.
MH