That's because your fanwheel blades have too much dust and fluff built up on it. Just take it out and and clean the fan blades from dust with a soft brush. If you brake the assembly, willingly or accidentaly, you can put it back together with a good quality sticky tape. After this a bit of vacuuming does help from the outside once a week or so to slow this dust accumulation. It worked for me and had to do it about 3 years after I bought it.