2007 Volkswagen Golf Noise and wont rev above 3000

I have just purchased a VW golf TDI 1900 on 2007 plate.
It has now developed a very slight whistle noise which is more noticiable when driving careful (low revs). It has also fails to rev above 3000 on occasions although not all the time.
I was adviced from the company that I bought the car off to take it to a nationwide centre, they placed a diagnostic machine on it but where unable to confirm what the fault was. (The young lad didnt seem to have much idea) but he did say it may be a leak from one of the rubber pipes to the turbo.
when driving the car it appears to perform ok other than this slight whistle