merc 300 td rattle?

R reg Merc 300 td, 46000 miles full genuine merc history.

glow plug light illuminated last week and car engine would knock for the first couple of minutes from cold, when warm would run great. changed all glow plugs the other day and all is now fine. what I have noticed now is that there is a slight rattle noise at around 1250 rpm. it goes when engine revved and it drives and runs great. it sounds like its an injector, I have been using super market fuel and wondering if ive got sh*tty fuel in the system. any other ideas what could cause it to rattle.