Q: What causes loud high pitched whistling sound ?

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My bmw is making a very loud high pitch whistling. It sounds like a siren. I took it to a mechanic and he replaced the valve cover but the noise still exists. Please tell me the problem. When I turn the car off the noise stops but when I turn it back on and drive for about 5 mins the sound comes back and it's very loud

My car has 170000 miles.
My car has an automatic transmission.

Of course it’s difficult to diagnose a noise without listening to it, but it sounds like you’re engine is trying to pull air through a passage that is not quite closing up. When the last mechanic replaced the valve cover, he was probably suspecting there was a restriction in the crankcase ventilation at the top of the cover. With a high pitched noise it can be difficult to get a fix on the direction that it comes from but here is a trick: get a piece of plastic or rubber hose about two or three feet long and put one end in your ear. If you wave the other end around, you’ll find that that it becomes a very directional kind of stethoscope, enabling you to pinpoint the origin of the sound easily. It will probably be a collapsed hose somewhere in the intake system, but it could even be something as simple as a loose gas cap. If you contact Your Mechanic they will send a technician to your home or office who can locate your noise and explain what you need to do to fix it..

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