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Blow off valves are designed to reduce stress on the turbo when the throttle is closed by releasing the boost. When you pull your foot off hte throttle, this creates Vacuum instead of boost and allows the blow off valve to vent the pressure built up between the turbo compressor housing and the intake.
This boost SHOULD be re-circulated back into the inlet pipe AFTER the airflow meter but BEFORE the turbo or else there's a high chance you can suffer from a "pop" on every gear change and/or a highly rich mixture between gear changes due to metered air being vented atmospherically and not entering the engine as expected. (assuming stock setup for computer etc etc).

You can get recirculating blow off valves that should be quiet, the stock bosch type BOV's are usually pretty quiet unless you're running extra boost, also having a Pod filter will increase the noise heard from the blow off valve.
 
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