Love that Registered Hypercam watermark
Searching "random steam noise in background" gives a result saying check to see if steam > settings > music is playing anything. Big picture mode also has sound settings but you need to be in big picture mode to access them.
It's not the music. It might be the big picture mode. I think I changed some settings a while ago thinking it was only active while in big picture mode. Currently I can't open it because it asks me to sign in?!? I will try to reinstall the steam client.
Did a download fail/you lose internet while in Big Picture mode?
I don't think so. I can only open it when I set steam to open in big picture and exit it completely. But I could not find any turned on sound settings there either. I have switched to the beta client maybe that helps? If nothing works I will just mute steam in the volume mixer.
Steam
For Steam Game Stuff