If it was grinding, that sound might have come from somewhere inside your sewing machine. DH cleaned my HQ16 after 2 years of medium use - about 20 quilts. It was immaculate inside - EXCEPT for one piece of thread that was wedged into the mechanism that raises the needle up and down. The HQ16 was running fine, but who knows for how long that would have been the case?
You can test whether it was the C&C thread or not by trying C&C again.
However, if your machine is an expensive one, I think I'd get it serviced, just in case.