Could anybody suggest the source of this whistle. It usually occurs when I slow down after a long run down the motorway of after it has been stopped and started. Its a Colorado 1998 KZJ95 that had 109k on the clock two years ago and now had 130k and has always had this problem. It does not do it on tick over and is usually between 1 and two thousand revs but at a constant pitch it is not engine rev related. It very rarely does it when stood still but appears to be round the fan area. If it was the fan, pullies, alternator or water pump the sound would alter with engine revs but it doesn't. It almost sounds like air intake whistle but is from the wrong area. I would like to get it sorted before I ship it to Canada next year for a grand tour. Anybody have any suggestions.