Have both the L and R monitors got the same problem? If so, the problem is probably the +ve rail of the power supply, which located in the L monitor along with the power amps. If the sonic problem is only in one monitor, the likely reason is an output drive transistor blown in that channel's amplifier.
Judging from your video, there is a short in the amp passing DC voltage.
That is why the speaker is sucking in.
I had a different brand do the same thing due to cranking them too loud, not that you were cranking them too loud.......
But that was the culprit
Have both the L and R monitors got the same problem? If so, the
Have both the L and R monitors got the same problem? If so, the problem is probably the +ve rail of the power supply, which located in the L monitor along with the power amps. If the sonic problem is only in one monitor, the likely reason is an output drive transistor blown in that channel's amplifier.