Hearing a Buzzing
 

Recently, my '87 300TD has developed a buzzing noise that I hear through the stereo (stock system). It is louder at the lower frequencies when in the FM mode. It does not seem to change in pitch as the RPM levels change though after awhile, it just stops. Awhile can be ten to thirty minutes. Though I suspect something within the alternator, I am not certain that is the only potential cause. All comments appreciated.
Steve