1999 Chevy Suburban catalytic converter

Our mechanic replaced our old catalytic converter with a new one from front to back up to the point where it meets the muffler pipe.

There are two problems:

Problem 1: there is extreme rattling. A mechanic in the town where we were when we first got the rattling said that the heat shield (which was part of the new converter unit) is rattling against the original heat shield on the car.

Problem 2: That same out of town mechanic said that there is a leak where the catalytic converter joins the muffler pipe.

I took the suburban back to mechanic 1 who is honorable. He says he will "fix it." I asked him how, and he said "I don't know."

Can you please tell me what to tell him?

Thanks.