I have found that when you hear your machine sound 'different' it is time to stop, and really work on your machine. I would take off my throat plate, mine(a Bernina) builds up lots of lint, from the thread passing through rapidly. Check my threading, my needle condition (if you have been sewing for 6 hours or the equivalent, change your needle) But I have also found my machine needs just a little oil. I put sewing machine oil on a cue tip and swab it in the bobbin case--without the bobbin. When your machine sounds different, it usually means it is complaining to you! I hear it as I need Attention! If you haven't spent some time oiling your machine, go over your manual and see what it says to do to oil it, or maintain it.