I have a 2016 Chevrolet Impala whose entertainment centre has a problem on AM radio stations. Periodically, mostly on rainy damp days, a strong heterodyning sound comes over my radio stations sound drowning them out. I experienced the exact same phenomena with my Blaupunkt radios in my Jetta's back in the day. A GM technician suggested that it was the shark fin antenna that was at issue. The Jetta's did not have that. They had a cowl mounted rod has an antenna. And they both had the same issue. Any suggestions, anyone? Sounds to me like an amplifier or preamp as the case may be inside the chain of the AM audio paths doesn't like High humidity. I have a M4A file of the sound but I can't add it here for you.