strange noise

[thewall]

I have a MK5 GT TDI,2005 30,000,i had the air con serviced by a VW dealership about a month ago and since then there is a noise which i can only describe sounds like the overrun which is what i first thought,but this noise is most noticeable when i'm doing about 40/50mph in 4th gear and less than 2000 revs,if i'm on a flat road my foot is barely touching the throttle,maybe the noise is always there but the engine and road noise at higher revs and speed masks it.
The noise sounds alot like a fan but not the rad fan everything else is working fine gears,wheelbearings and engine performance.