Audio Systems: humm when connecting tv to receiver, fuse panel, pc speakers

Question
Have new Denon AVR 1908 Receiver.  Have Cable, DVD connected to Denon via HDMI In.  Have Denon connected to new panasonic 42" TV thru HDMI Out.  
Use to have PC (for MP3) hooked up to old TEAC via A/B switch (can get sound via PC speakers or Receiver Speakers) - no problem.  Tried same hook-up to Denon and immediately getting significant humm on system.

Answer
It initially sounds like you've got a ground loop issue. Here are some things to check:

1) if you are using "directional" cables - chuck them. If connected improperly, they can severely damage your system, and there is no such thing as a "direction" when it comes to audio cable. Audio is an AC waveform - it travels both "directions".

2) Check your electrical outlets. Are you split up across different circuits with your gear? In some houses (usually older homes), if a receiver is in one circuit and the TV in another (examples only), you can get a ground loop. Try having everything on the same circuit (not necessarily the same outlet, just the same circuit from the breaker / fuse panel). This is especially common with powered sub-woofer as they are not always near the rest of the a/v gear.

Without seeing the system personally, this is about all I can think of. I hope this helps.