Popping sound from speakers

Hello,
I have a PA system and separate speaker system with two medium size/power speakers that attach to a control box. I use this for singing/entertaining at small venues. A couple months ago during a performance, the speakers both started making loud popping sounds; I had to turn the system off. After a brief intermission, I turned the speaker system on again; all was fine for a few minutes and then the popping started again. I could not determine the problem. I used the same system the same way 3 times since in different locations without anymore problems, making me think that some interference at that one location had caused the initial problem. Tonight, I performed at a location where I had performed a couple months ago with no problem. But tonight I got the popping sounds after the system had been on for about 15 minutes. I turned the system off for a while; turned it back on and it was ok for a few minutes before the popping sounds started again. Once they start, they don't stop till I turn the system off. Each time I turned it off for a while and turned it back on, it was fine for a few minutes, but the pooping would return. Any ideas? This seems to happen whether or not I have the PA system hooked to the speaker system. Any ideas?

Amp going?

The cell phone possibility is something I'll check out. At this evening's event there were 3 cell phones in close proximity to the speaker system control box. If this is the answer, though, I wonder why the popping wouldn't start as soon as the system was turned on. Why would it wait till after a few minutes?