I have recently bought a Focus 2.0TDCi 06 plates, 92000 miles and last service at 88000.

Have discovered that whilst accelerating in 4th,5th gear and sometimes 3rd, on reaching 2200rpm i get a strange faint high pitched whirring with a very slight dip in revs down to 2000rpm with a slight loss in power, almost unnoticable but there all the same. It normally gets better and carries on accelerating again but in 5th gear it can happen again at about 2400revs dipping to 2200 and carrying on as normal. I then noticed that it also happens on the motorway in 6th gear and always at 2200-2600 revs. No warning lights come on and also there are no other probs i can see or hear with the car.

Would love to know if anyone has any ideas what the problem could be? I am not very good at DIY cars but would like to know before going to a garage if it may be pricey or easily sorted.

Thanks