I have a semi-intermittant "ting-ting" type noise that I believe is coming from my exhaust, midway along the vehicle. It sounds like a loose piece of metal, but there's nothing external that's visible. I'm thinking something has come loose inside one of the catalytic convertors. Does this happen? If so, is there a fix?

Any other thoughts as to what this could be?

Also -- related to a Mitsu Diamante. Can overfilling the crankcase blow a rear oil seal?