Mine did the same thing when I disconnected the the plug that leads to the GPs. I not 100% sure what I'm talking about. I changed my GPs (300D Turbo) and was taking off the relay to open it and clean it. I think I got the buzzing when I applied power but didn't have the bigger of the two plugs attached to the relay. I do recall a very loud buzz as the relay vibrated. Once the whole thing was back together all was normal. Might want to clean all the contacts inside the relay.