My building has lousy grounding.
The external cable grounding on my building {water mains} had been there so long the clamp and the cable were oxidised to the max. A new clamp cost me about 10/15 bucks. Had to cut the old clamp off for the new one. Another problem with this is they have been changing all copper piping to PVC. So could lead to False ground connections.

I also added new receptacle{PS Audio} with an extra ground line to the receptacles box. For me these made a great difference, especially on my noise floor.

Sadly there's no easy fix for such issues. Only process of elimination. Sometimes multiple fixes are necessary.