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My approach would be to see if it's possible to remove the auxiliary drive belt and start the car from cold and see if the noise disappears - this will either rule in or out quite a few components.
Any half competent garage ought to be able to do this. I'm not familiar with the N13 but on my old 530i you could easily disconnect the belts by turning the tensioners with a socket
Hopefully the noise disappears when the belt is disconnected, if so it's going to be alternator/ AC compressor / tensioner or possibly damper if there's one fitted to these engines
It's a tricky thing to diagnose so best approach is at least to eliminate the easy things first
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