[MargaretR]

I never managed to get in it to clear out dust and cat hair.

I did get as far as disconnecting all the spaghetti at the back, but was flummoxed at getting it open.

I reconnected the spaghetti and went onto Dell website to read the online user manual, so know I know how, but I think I have found the cause of that grating/pipping noise-

I had been getting an error report message which was connected with 'Kontiki' (thought that was a raft!!)
Eventually identified it with that BBC download programme - so uninstalled that and have had no problems since - so the dust can stay put a bit longer.