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To prevent the presurized air that just got stopped by the throttle from going backwards through air mass meter and causing over-richness (amm can't tell which way it blows, it would register air motion and fuel computer would oblige with more fuel unless throttle was released fully) there is a device called the bypass valve that opens on sudden vacuum increase to eliminate this problem. Is has that side effect of creating that gasping noise. Volvo put foam into air cleaner box to cut down noise but the foam is restrictive. Noise good, restriction bad.
This valve also helps to keep the turbo from losing so much speed when you lift for an instant as it opens a recirculation path for the air.
On 84-89 B23/230FT this device is external on braket and is plumbed into oulet and inlet hoses at turbo. On 90 and up it is integral on compressor housing of turbo.
If this noise really bothers you the solution that is best is to get the non-expanding spray foam (the expanding kind has a nasty habit of making dents when placed into closed body cavities, don't ask me how I know this) and put it into fender just in front of strut tower and above the inner liner/wheel housing.
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