It's just blowing out a little ditrus that you missed. Could be in the motor or tubes.
You might not be cleaning it well enough or often enough. Set up a 5 gal bucket and restart your fluvel in that first after cleaning it. Yes things will start to collect in the tubes if you let it. Get some plastic coated wire, tie your wire around your apropriate bristle tube cleaner and thread the fluval tube, draw it through, rinse. It's like cleaning a gun.
BTW loop the wire so you don't poke a whole in anything.
Thomas