Have you oiled it recently? Does it need greasing? Some machine need both.
Also, have you cleaned out in/around the bobbin case?
Have you run dental floss through the tension disks to see if there is anything caught up in there?
I'm not very experienced with all this, but it sounds like something to do with the top thread - follow the thread path slowly and thoroughly, and if possible, thread only partly, run machine and see if you can tell where the noise starts up or originates from?