Why is there a humming noise in my sofa?

There's a leather sofa in my music room. Sometimes, I lie on the sofa to listen to music. Last night, I lay my head on the arm, and when the music shuts off, I hear a medium pitched hum that has a slight pulsing to it. I turn the music system off, the hum is still there. I just checked again, hum still there.

There is nothing in the room but the shut off stereo, my piano and drum kit. The room was built for acoustics and noise reduction, so no sound should be coming in the room from somewhere else.

I thought it might be like the sound you hear when you put a seashell to your ear, but it wouldn't be a pulsing hum. Any thoughts?

Dad was at my house last night and I tried to get him to listen, he said "I'm not listening to your couch or anyone else's."