I had exactly this problem with my M4-P. Although the effect on the negative was a little different which may be because I've darkened the attached image to make the problem more pronounced. It only showed up with faster shutter speeds where the gap between the curtains was narrow and being almost spanned by the fluff.
Originally Posted by Peltigera
Holding the camera at an angle with the back removed and the shutter held open on B, I could just about see some tiny strands of loose felt attached to the metal strip (cap?) on the edge of the curtain towards the bottom of the camera. A combination of tweezers and a bit of huffing and puffing removed them.
Hope this helps.