The most common problem with your cars speakers making strange noises including whistling, screaming , popping is a fault in your BM54 radio/amp module made by Becker.

Some part numbers for these include:
65.12-6 933 455 65.12-6 976 964 (65126976964), 65.12-6 964 400 (65126964400), 65.12-6 964 401 (65126964401)

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