Ground Loops are tough to ferret out!!! As an electrician I'm letting you know where I'd start. I tseems to me in most cases ground loops are caused by interference placed on signal cables. I.d check the grounds as stated to eliminate the ground ware as the culprit. I've seen waires installed on the ground bar looking like they were placed correctly only to find out they were resting behind the ground bus!

I've also seen where it is a faulty piece of equipment injecting the noise on the signal wires. Jim