Hi Kenneth 兄, I am also Kenneth.
First of all, congratulations! Second, I also encounter the same hum sound problem as you before. I do one thing, then it is totally eliminated. That is to cut the ground from the 天線插 on the TV receiving side.
Assume you are using iDTV:
Wall (天線插 A) -> 天線 -> (天線插 B) TV
What I mean is to cut the ground cable inside "天線插 B". Just leave the core wire and separate the shielding wires with the 天線插.
In my case, I have no signal degrade. I used to have a signal splitter wall socket that outputs TV and FM, but I removed it and connects the signal cable from wall directly to my TV. The signal before (with splitter and grounding connected) was about 40%, and there was lag occasionally when I watch Jade HD. However, after removing the splitter and grounding, there is no hum sound and the signal goes up to 70-80%.
I think in my case, the improvement is due to removal of the signal splitter. I don't know how is your settings. You'd better tell us more about your cable topology and hope other c-hings could help.