Just finished disconnecting the inputs and outputs from the 990 and 7700. Just in case I also disconnected the speaker cables to the amp.
I disconnected each cable one at a time turning on and off the 990 and 7700.
End result was that the hum is still alive. I also checked ALL of my other connections to make sure they were correct and tight.

I have a couple of options.
Input is appreciated.

First, I can disconnect the amp take the cover off like XM suggested and take a look to see if there is anything visibly lose and perhaps use something like a pencil to prod around the guts of the amp.

Second, There is a electronic repair shop not to far from me that over the years has done repair work on a Yamaha receiver and recently did some work on my ten year old Sony 777. He is very reputable. Since the unit is out of warranty it would save me the shipping and handling fees as well as not worrying that the unit could sustain more damage thru shipping. As you know the unit is 93lbs.

Third, I could do nothing and when the unit on channel stops working take action to repair it at that time. The hum as I have mention does not interfere with the enjoyment of listening to music or watching a movie.
What if the idea that the voltage in out out of the house is the issue? Then I waste my time and money bringing into the shop.

Thanks guys and looking forward to a response.