I only say that as you say it happens after you have sat with the clutch pulled in for a while as when the clutch is pulled in the plates are free to move about and if the top steel is too thin the top friction plate is more likely to ride over the top of the crown wheel when the clutch is pulled in!
it then wedges between the top of the crown wheel and the top steel which makes it drag, the grinding noise could be the ears on the friction plates scraping and dragging across the top of the crown wheel till it gets to the next notch and falls back in to place.
PS: is there a prize for who ever gets the diagnosis correct?
