Break in the chatter

I TAKE it the chattering is a relatively new development, in which case it suggests something has changed. It's difficult to diagnose your problem, but I would be looking closely at the discs and pads. I would look for signs of glazing of the pads, which can happen on new pads if they're not properly bedded in. Glazed pads can cause the sort of chattering you're experiencing, and I'd also consider having the discs machined to make sure there's a good surface for the pads to work on.