I posted a few months back about this problem and have narrowed down the
root cause. The symptom are an overpowering hum at the low end of the am
band. It's being caused by a 3-phase distribution line. I can follow this
line in any direction for several miles any time of day with any of my
vehicles and get the interference. I contacted the power company and
supposedly they're going to look for the cause. Has anybody else dealt with
this type of problem?