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My other half has just phoned me to tell me that her Astra turbo Diesel has developed a hissing noise from the engine that sounds like a leaving tyre.

Apparently there is no loss in performance and no noticable smells.

The engine has recently been rebuilt with new cylinder head and cambelt.

is it a noise we should be worried about?

Is it the turbo? Mine hisses a bit, only just noticeable, and I am told this is entirely normal for my engine. Could even be wind noise, that she hasnt noticed until recently because its become very windy?

Sounds like a boost or intake pipe has come loose..

I was thinking it might be something like that, nothing to do with wind, as it is doing it when stationary. Not heard it myself yet, will have to have a listen

check all pipe work for splits, loose connections. also if the car smokes more than normal it will be an air leak.

Does it only do it while the engine is running?
The little switch under the heater/blower controls is operated by vacuum from a pipe that connects to the inlet. It's a pathetic plastic pipe like a drinking straw and I've probably broke about four of these.
Push the switch in and see if you can hear a 'click'. If not, the pipe is either broken or a rubber pipe has come off!

You can start the car and stop it and push the switch in. If it makes a clicking noise from behind the dashboard, it's okay.

Makes me think the mechanic or whoever did the repair has left something unattached or broken this straw pipe.

Oh, am not sure if this is the same on diesels, but if a vacuum pipe is leaking, you'd notice with the idle being all over the place! The bigger the leak, the worse the idle gets.

I have found it, one of the pipes that runs from the water bottle to the radiator is split.

Good job you found it! Was it water or air?

Water.

Just need to get hold of a hose now.

Turns out it was a pipe that goes to the water pump, unable to get hold of one, so have cut it, put some copper pipe inside and used jubilee clips to attach it back together.

Seems ok so far.