I once had a student who's machine was very noisy and producing less than quality stitches. We changed needles, re-threaded the machine, checked the bobbin, cleaned around the bobbin area, etc. but with little change. Finally, I asked if she had her little screw driver so we could take the plate off and she looked at me funny and said, "I always wondered why that screw driver was here." Needless to say there was a lot of lint build up there and when I removed it, it looked like a mini felted wool blanket!
The machine sewed beautifully after that, but the student was not quite happy. When I asked her what was wrong, she said that the machine was now too quiet and she kind if missed that familiar rhythmic pounding sound. She got used to the quiet and now regularly cleans her machine. We still laugh about that. :-)