The Road King has a lot of tubes (6 power tubes, 2 rectifier tubes, and 5 preamp tubes), so it gets hot. To combat this, it uses a fan to help cool the amp. The fan doesn't make any sound through the speakers, but it does make "a persistent hum" that is audible when not playing. Could it have been the fan that you heard?Thanks for the info. The amp definitely had a great heavy sound, but there was a persistent hum even on standby. Learned that it was bought used from GC. Walked away from that one. Hopefully I have better luck with the head unit.