VFDs need really good grounding. Make sure you have a solid earth ground or you can get arcing across the motor bearings. This makes a sound that’s often described as “fluting,” and I think that might be your problem. If it is, you need to fix that before your bearings are trashed and you have to replace the motor.
Wait, the compressor and drive is a device or a pair of devices right next to each other, assembled in a factory, and made out of metal. Why does the quality of the connection between the hunks of metal and the planet have any effect?
VFDs can produce nasty waveforms, and there are cases where “grounding” could be a big deal, but I think that the wiring of the ground terminal of the power supply is only relevant at all when it’s involved in the connection between the drive and the motor. So, for example, if you have a VFD that is far away from a motor, then you would want to make sure the VFD and the motor’s grounds are connected to each other and maybe even that the VFD’s supply neutral (average of the phases) is reasonably close in voltage to ground, keeping in mind that there may not be an actual neutral wire connected to the VFD, and that the motor’s ground is well connected to the VFD’s ground. By modern standards one should use actual VFD cable and terminate it properly.
The outlet are grounded with a thin non insulated copper wire secured to the nearest water copper pipe, itself also bounded to the iron gas pipe (this is 1950 electrical). I am not sure I can call this a solid earth ground.
It isn't bonded at the panel. The panel ground is not used on any of the 1950 circuits. Neutral is bonded to the panel ground. And yes this does means neutral and ground pins at the outlet have a few volts across, since the path between them goes through the literal earth. The water and gas piping coming out of the earth at the opposite side of the house from where the electrical panel is located.
