We have had this problem before but it fixed itself. It's a DC10 (purple and green).
It only happens when the roller is going round. I've cut the tangle of nasty stuff off that's around the roller so it's perfectly clean and checked all the pipes.
Any ideas?
I don't want to have to drag it down to a repair shop (not that I would be able to find one anyway) and the carpets are getting compacted with yuk.
Even if you just give a wild guess, do please share it.