I occasionally get a slightly gritty sound for a few return strokes (only for the first part of the stroke), on one strop with one razor, each time i heard it i had a look at the razor, wiped the edge on my palm and wiped the strop with my hand.
I didnt feel or see any foreignmatter on either, anfd it went away on its own, the only thing i can think is that a small piece of dry skin, hair or or other shaving debris was stil on the razor.
It shaves fine, the strop shows no new damage so i didnt wory about it too much, though it will be interesting to see the responses, possibly trimmy's idea could play a part.