Speaker makes a popping sound

I have a PA system that I use with a speaker system when my daughter sings/entertains at various small venues. Both systems have worked fine for 3 years. In December, during a performance, loud rapid popping sounds started coming from both speakers. I had to turn the speaker system off to stop the popping. Even when disconnecting from the PA system, and turning the speaker system back on, the popping sound would start again after the system was on for a few minutes. Both speakers are connected by long cables to a control box. For the shows, I connect the Speaker control box to my PA system which plays the music and has its own set of internal speakers.
Since that first "popping" experience, I have used the system 3 times in different venues with no problems, leading me to believe it must have been some kind of interference at the venue where I had the problem. Tonight, we had a show at a different venue, but one where we had performed before with no problem. After about 15 minutes of performing, the popping started again. After turning the speaker system off for 5 minutes or so, we turned it back on and all was fine for about 5 minutes; then the popping begain again. It seems that each time I turn it off for a while the popping sound comes back again after a few minutes.
When I got home, I turned on the speaker system without the PA system connected - or even in the same room. After about 10 minutes, the popping started again. So, it's not the venue, or my PA system creating a problem. It must be something with my speaker system. Any idea what it might be or what I can do about it?