I have now tried Steps 1 to 3. I am not clear on what you mean in Step 4 and I have no way of knowing if the ground lugs as connected to the house ground... But the house was built in 2001 if that tells you anything.
A house built in 2001 SHOULD BE up to current electrical codes for grounding... plus or minus unknown variables like the integrity of the builder and local building inspectors.
Two things you can check. For safety, do these in the right order.
1. To determine whether there is any voltage on a ground lug, use an AC voltmeter set to a range 220 volts or higher to measure whether there is any potential between ground lugs on two different AC outlets (not just two on the same fixture). If so, you have serious wiring problems.
2. Only after you check for any voltage on the ground lugs, set the meter as an ohm meter to see if the ground lugs on two different AC outlets are connected to each other. If they are, you should get a very low reading between them. Theoreitcally, it should be zero ohms, but even very large guage wire has a finite resistance.
So far the only thing that stops the sound is unplugging which ever optical cable is being used for the current source but then of course I lose the sound altogether.
Hmmm... Optical connections are electrically isolated so they shouldn't have grounding problems. Another possibility could be that the clicking sound is a "beat frequency" between two high frequency digital clocks oscillating at very close frequencies. The rhythmic clicking would be at the difference between the two frequencies, and the closer the two frequencies are, the greater the amplitude (loudness).
As I noted, above, you could get the same kind of beat frequency from an interaction between the local clock in your receiver and the clock in an unrleated, nearby electonic device such as a cordless phone. When I talk with one of my friends on the phone, I sometimes hear a click-click-click that appears and disappears as he walks around his livingroom.
Most digital gear is designed to sync and lock the clocks between external sources and an internal clock, but this could happen if the sources drop out of sync. Have you tried connecting the same external digital sources to another system? If they work with another system, it would suggest that the problem is in your receiver.
If it's not impractical, you could take your receiver to a friend's house, and test it with his/her sources and AC power.