Why does my low frequency speaker have static noise?

If only statically during audio playback, check the internal speaker wires to see if loose connection, stray wires are causing the problem due to the vibrations of the speaker box. Also check your audio input cables to the unit. Do a visual inspection of any circuit boards for cold solder joints, cracked solder joints, overheated spots, bulging capacitors etc.

If static is present with no audio signal applied, then using a signal tracer or oscilloscope to trace through the speakers circuitry should be of assistance in tracking down the problem.

If static begins when hooking up your external PA devices (ie mixer, mics, mic cables etc) then they will also need to be checked one by one to eliminate them as a possible source.

The following may also be of use to you:

https://old.reddit.com/r/audiorepair/