A good trick to deal with this problem is to run your whole rig to a single ground (including mics). This is easiest to do if you get a grounded serge protector / power conditioner and run the whole rig from it (making sure you are not overloading it).If you make sure that everything is grounded to the same wall socket you will have a quieter rig as you have eliminated ground loops which cause hum and pick up radio frequencies.