I have had problems with a gradual (within 1 week) degradation of AM radio reception. Lots of crackling and whirring.
This happened about 18 months ago to my 2001 330i. I took it to the dealer, who them charged me $150 for electrical work and said nothing was wrong with it. Interestingly, I had good reception after I got it back. So, I reasoned it must have been a simply loose cable they removed during the diagnostics and reinserted and subsequently tightened.
Then it happened again a few weeks ago, and I was not going to pay the dealer to tighten cables. I read in this forum about the antenna amplifier and reasoned the connection to this box might be the problem. I removed the opera light in the left rear seat (tricky), then removed the panel (tricky) to expose the amplifier box. I removed the coaxial cable (it was not particularly tight) and reinserted it and used some pliers to tighten. I removed the other wire connections and reinserted. Reception is back now like new.
I imagine this must happen to other cars as well given enough time for the fasteners to come loose, and some possible oxidation over time of the contacts. So, I wanted to post this for others to see and ask if anyone has also had similar experience and fixed.
I have not precisely pinpointed the exact connecter, but suspect the coaxial cable connection leading to the radio since my reception went bad on all rear window antennas, and I could hear switching of the amplifier between antennas trying to get the best signal. Anyway, problem has gone away, maybe for the next 18 months.