I had to have my PC6500 serviced for the same thing. Threads kept breaking and I ignored it, then thread started to wind around a bearing in the tension/threading assembly. I had to have a new assembly put in. I asked what caused it - thread, after so long starts to wear a groove in the bearing since it is hard rubber or plastic. Once that happens...
Hopefully it just needs a good cleaning. I must say, though, that since I had the machine in for repair and a spa treatment it has never run so quiet.