I own a pair of Monitor 7Cs bought way back in 88. At times one of the speakers makes a buzzing/vibrating sound that is more or less continuous for a period of time before cutting out. The sound does not appear to be dependent on the volume. At other times, there does not appear to be any problem at all. Is the midrange blown? Or could it be the subwoofer? How would I know for sure? Is there a test I can perform to determine which of the drivers may be faulty?