Little update, took it into work today and the boss said he has exactly the same problem on an old Audi he has. He said it was a pulley not being lubricated properly, and told me to squirt a little DW 40 in behind the pulleys. And so far it's gone. It got a lot better when warmed up but not seems to have gone completely. Will keep an eye on it though. Cheers for your help guys!!