I have a S.E.X kit with C4S that I built a few months ago and it was perfect! However in the last week for some strange reason the left channel pops and hisses for the first 30 seconds to a few minutes whenever I turn it on. It doesn't pop or hiss after a little warm up, but the period for which it pops and hisses varies every time. I've switched tubes, headphones, cables, and sources to no avail. Plz help