That tends to happen on my machine, especially if I'm working it a bit harder. Every now and then, I'll be working on sewing projects for an upcoming Craft Show, and can spend a few hours of non-stop sewing.
Here's what may be happening. Lint begins to build up underneath the Stitch Plate and Bobbin area, each time you sew. A lot of lint does form when sewing with Fleece, Terrycloth, Batting, and anything else that is "fuzzy".
My suggestion is to simply remove the Stitch Plate and open the Bobbin "door", and remove any lint balls that you see, by using a small brush or tweezers. Do this often, and your machine will soon be humming a happier tune.
