You will probably need to get a knowledgeable person to look at your setup. Any time you have more than one ground connection, you will get a ground loop (may be inaudible).There is a small possibility of it being caused by an equipment fault, but unlikely.
You probably have 3 grounds – Vox+Vox+PF. But, you don't want to go around disconnecting grounds without knowing what you are doing. For now, make sure that they are all plugged into the same powerstrip, with mains leads as short as possible, ideally tied together. This may help – if not, you will need to find a tech to look at it.
This problem can occur whatever equipment you have – it's a matter of how it is all connected.
