10 yo monitors; could be dry solder joints on the PCB, that is if they are self powered; IMO; dump them and get a pair Behringers 2031As or something similar; I have 36 of them in my VTPO installation.
Have a techie inspect it and check the wiriing and PCB for dry solder joints. Thumps, coud be faulty output transistors or the muting relay not functioning.
just my 2cs worth.