Slight humming noise coming from Mark Levinson 331

I was changing an LP last night and noticed a very slight hum coming from my amplifier. I have owned this amp for about six months now, and just noticed the noise. It is not noticeable while listening to music, but you can hear the faint hum from a few feet away provided the room is super quiet. Is this normal, or should i be concerned? I bought this amp used from a authorized Levinson dealer and have not had any problems with this amp at all! Just wondering if this is normal, or am I going to see smoke and flames in the near future? There is no buzzing or hum from the speakers at all, just the amplifier. The amp is plugged into a Transparent Power-Wave 10 power conditioner. I have tried plugging the amp strait into the wall and it still makes the humming sound! Thanks for any input on this issue!