Ducato Hello and help

[royale10]

I've read a lot about the mooing sound (probably herd all the jokes as well) but has anyone any ideas of a cure on a 1994 2.5L turbo diesel Ducato. All is quiet until about 15 minutes after stopping when the noise of a cow or fog horn comes from the front of the vehicle.
I've seen the blame put on the pressure relief valve (whereever that is) but has anyone any specific advice.
Cheers
John