I have a very peculiar thwapping or whizzing noise that only lasts for about 5 seconds once I let up on the gas. The noise decreases in frequency and amplitude and almost seems like something is slowing down and stopping. I can hear nothing any other time and the car seems to be operating very well- There is a slight hunting for idle at cold start for about 30 seconds. 1992 940 Turbo 106,000 miles. If you place your tongue on the roof of your mouth and make a sound like a weed wacker then you have a close proximity. everybody try!