I have the group loop problem before when connecting the CABLE TV antenna cable. I bought a Jensen isolation transformer and the problem solved. So you found the ground noise, you may try to unplug the cable TV antenna to see whether the problem is come from there
海記, I bought it from eBay, but the shipping cost is big when you only purchase 1 unit. So I bought 2 units and having 1 unit left. So if you need it, I can give you to have a test before you pay for the cost (USD110/2=USD55)
In fact, there were only a few "hum" coming from the TV previously.
However, they still exist some times (less than before) especially when watching the football matches. Don't know the reasons...
Anyway, please call me when you are free; or;
would you please tell me your a/c by email (?) so that I can transfer the money into your a/c ?