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Q: It knocks once after i start it up only one time and when i'm drivin a charm is played kind of like an alarm what should i do?

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it knocks only when i turn it on, one time and the knock is sometimes soft or hard and other times it doesn't even do it at all weird

My car has 160000 miles.
My car has an automatic transmission.

Hello there, a knocking sound can cause problems if left unresolved in your 2008 BMW 535xi. A knocking noise can be caused by a few different faults such as an engine mount, engine knock, failing alternator, or AC compressor. If you have an engine knock this needs addressed immediately. If you need assistance, a qualified technician such as one from YourMechanic will be able to diagnose your knocking noise and determine what the cause is.

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