I've found things using switch mode power supplies, internet routers, cell phones, TV's, and even newer refrigerators can inject noise in a system if to close to audio equipment. Especially preamps and phono amps.
But the noises are usually clicks and various noises and tones.
I've found hissing noises can be caused by a noisy transistor, or the occasional capacitor. Being that its at regular timed intervals it could be a cell phone or router maybe. Noises do happen with inputs with nothing plugged into them though. Usually a hum of some sort. Especially phono inputs. Thats why some audio companies would include shorting jacks for phono inputs. Especially if there were two phono inputs and one wasn't being used.
Most of my equipment is tube based and not having something plugged into amplifier inputs can produce a hum.
Personally I would eliminate the possibility of outside interference first. Turn things off around the house one item at a time. Like cell phones, routers, etc. Including any wall warts that tend to use switch mode power supplies.
Compare electeolytic capacitor orientation between the two channels.
Also look closely at all your solder joints in the offending channel, especially the grounds. Reflow any that look a little dull.
Good luck!
