Really odd situation with my sewing machine.
I use an older Bernina to piece. Yesterday it started to make a thumping sound. I changed the needle, cleaned out the bobbin area and stitch plate, put in new bobbin, and rethreaded. Still made the thumping sound. I realized the only thing different I had done was to change the brand of thread I was using. I thought that can't be it, both are 50 wt cotton. I threaded the machine with the thread I was using before and no thumping. Threaded the other thread and thumping started.
I did it three times and using one thread no thumping and the other thread thumping. Any one have any idea why?