One of our first rescue and repair machines was a Dial 'n Sew. A simple ZZ machine made in Japan. It's always been sooooo quiet it's a pleasure to use. I was happily sewing along piecing some squares when just all of a sudden it started clunking. I stopped, looked at it, put in a new square, sewed it ..... clunk clunk clunk ...... WHAT THE Hxxxx????? So I started looking, touching, oiling and gradually eliminating things. I knew it had to be in the hook drive somewhere.
Finally I took the hook and bobbin out and it was quiet again. Hmmmmmmmmmmm ??????? It wasn't that dirty. But I did clean it and oil it and put it back together. All quiet again.
Some machines are just more touchier than others I guess.
Joe