This is acoustic feedback. Just because the frequency is too low for you to hear doesn’t make it not feedback. The trigger could be your footfalls or environmental sounds but that can be enough to set it off.
The simple solution is a rumble filter which will cure it for sure. Otherwise you need much more sophisticated isolation. You did not say how far you can get the table away. I’d say a minimum of ten feet and ideally more. If you are six feet or less then you are making the problem harder to fix. Real isolation solutions for subsonic frequencies are very expensive (think thousands of $) - space and air is much cheaper!
The simple solution is a rumble filter which will cure it for sure. Otherwise you need much more sophisticated isolation. You did not say how far you can get the table away. I’d say a minimum of ten feet and ideally more. If you are six feet or less then you are making the problem harder to fix. Real isolation solutions for subsonic frequencies are very expensive (think thousands of $) - space and air is much cheaper!