Fiat Punto Engine Whistling?

I have an X reg punto that's done around 75000miles, its had relatively few problems through the 10 years, but recently had to have the head gasket changed. When we got the car back from the garage after having the work done we found there was a faint whistling. We took it back and the garage said they found nothing (if they checked at all - we have gone elsewhere since). More recently the whistle has been getting louder and louder. It seems to happen when the engine is roughly at half temperature and between 2000 and 2500rpm in 2nd and 3rd gear. There is no real problem with this (no warning lights, no loss of power etc), its just very annoying. I have checked all the connections I could find for things like hose connectors etc and all seemed tight and yet no improvements. Any ideas would be great.