PA system making loud noise

Our Yamaha portable PA system has started making this really loud buzzing noise when you turn it on. You have to turn it off & on several times before it'll stop. We've taken it in to be "fixed" a couple times but then it always starts again. Can someone tell me what might be causing this, and if it might be more cost-effective just to get a new PA system. I know very little about audio systems.
 
Our Yamaha portable PA system has started making this really loud buzzing noise when you turn it on. You have to turn it off & on several times before it'll stop. We've taken it in to be "fixed" a couple times but then it always starts again. Can someone tell me what might be causing this, and if it might be more cost-effective just to get a new PA system. I know very little about audio systems.
1) *Which* "Yamaha portable PA? There's thousands of models.
2) Hard to tell you what it is with such little info
3) it should be repairable, of course, but by somebody properly trained and with access to schematics and parts.
4) *where* are you?
Different situations in different Countries.
5) so called "intermittent" problems are the worst, because they often are not present while testing.
You can't detect and repair "what is not happening".:(