I hesitate to even weigh in on this but some jurisdictions don't allow for a second grounding point in a buildings electrical system. It creates two points of reference to ground and can create ground loops! Not saying that is your issue..The panel is grounded to a large copper rod in the ground.
For example if a shop has a main panel it will have a ground buss that should have a ground rod. But if there are sub panels through the same shop, they should not have additional ground rods.
The idea is that all paths to ground have near zero potential.
One test I have people do when they are having noise issues is to take a volt meter and measure the AC voltage between the ground side of a 120 plug and the safety ground lug and see if there is voltage. I've seen 3 volts and higher in some cases.
But, if you are not using any 120 volt circuits in your cabinet then this is a moot point. Depending then on how your 240 volt circuits are safety grounded you can still induce stray voltages from bad ground systems.
It just a thought and may not be your issue at all. Talks 30 seconds to do the test!
Dave C.