If it's a black and white print you can always knife it and spot it. Probably not fiber tho.
I usually look at the shutter curtain, etc., after taking a roll out, and, with regard to said camera, I do not remember seeing anything amiss. I had said camera cleaned and checked before I began using it.
The best lab I know of is the one in your house.
I know how hard it is to get everything going (I've done it every time I moved for the last 30 years), but it's worth it if you can.
Could be some dust in the camera, just floating around not having much to do except piss you off.
Try a soft brush for the inside, under the mirror too. Then a blower.