I have a behringer eurolive b115. This is a 15 inch active 1000w speaker. ie has its own amp in the speaker.
When I turn it on all the light light up as normal then after 2 seconds the speaker squeaks then the light go out briefly then the process repeats. Lights on, 2seconds then a squeak, lights out.
There is an earthing screwed to the rear panel of the speaker. Around the earthing there is burn marks. Only Accessed by removing the rear panel