solid state power amps

Have  had my BAT VK-600 power amp now for 5 years, bought second hand from a dealer., and am very happy with its performance. My questions is this, when I turn the amp on it make a slight sound. hard to describe- sort of a low buzzing sound or the sound  liking of a transformer. Even when the amp is warmed up the sound is still there. When playing music, even at loud volumes, this sound does not come thru the system and is not amplified. Is this sound I'm hearing a general characteristic of a solid state amp. Any help or advise will be much appreciated.