Jim, I can think of two things that might cause your problem. One is that your clutch disk has become contaminated by oil which will cause the chattering, or you have a problem with your throw out bearing.

When I got my truck the PO had an odd throw out bearing in there that didn't work well with the clutch fork. I changed it out for a new one and my chattering went away.

Someone else may have other suggestions but I would try replacing the clutch disk and throw out bearing.

Good Luck
George