I have not tried either suggestion XM.
I will attempt to disconnect as you said input and output.
I am a little nervous about taking the cover off the 7700. Is it a simple task? Any tricks to putting it back on? I am sure you have heard that it is "easy to take something apart but to put it togather is another".
I won't have the time to do all of this until Monday.
What should I look after I take the over off?
When disconnecting the inputs and outputs if I find the noise stops what then?

Thanks a lot XM for your help and patience. This issue is really frustrates me. Otis so small but so annoying.

Frank

First go to Autozone and borrow or buy a mechanics stethoscope. It is basically a set of earphones connected to tubing which connects to a probe. Cost is like $10. This will help you localize the hum to a specific area of the amp. The cover is held on by several screws and when they are out you can just slide it off the top and remove it. Make sure the amp is unplugged for an hour or so before you take the cover off. Once the cover is off do a visual inspection to see if it is something obvious. If nothing is found then use a nonconductive probe to genttly push around things like heat sinks circuit boards and other hard mounted items. Don't poke at anything soldered to the circuit boards and do everything gently. If you want to see what it looks like before you remove the cover there are some pictures of the amp without the cover on the forum somewhere. I will try to find them.

With your system off, disconnect one channels input and output and then turn the system on to see if the hum is still there. If it is turn the system off and then move to the next channel.

Here is a link that Gonk made a while back to pictures of the inside of the 7500 amp. The 7700 is likely going to look like the 7500 but without the space between channels. Sorry could not find any of the 7700 but Outlaw used to post pictures of the inside on the site. Ask them if they have any of the 7700. Bear in mind that your amp is still under warranty and a call to Outlaw may be helpful. I have read about the mechanical hum on the forum before so they may have some ideas.

http://www.prillaman.net/outlaw_pics/

XM
Thanks for all of the tips.
I WILL DO EXACTLY as you say and follow your instructions. I most likely will be doing this on Monday of next week. I will get back to you via this forum by Tuesday and hopefully I can tell you I found and solved the problem.
If I do find the problem by doing what you have suggested I will owe you a beer!
Ugh that amp is so heavy!
Thanks again
And please check the forum on Monday or Tuesday for the results.

PS I don't believe the amp is still under warranty. It's better than five years that I have had it.

Well if the amp is not under warranty more reason to BFC!!

Absolutely!!
Yesterday and today...no hum!
Last night...hum?!!
WTF.

Normally I would not think of electronic equipment, which does have any moving parts, as being subject to vibration but.. transformers are known to hum sometimes. It could be a loose mounting screw or an isolator that is not performing well. The transformers in these amps are very large and need to serious mounting hardware so it is possible that is the issue. Once you get the cover off and do your inspection, if you don't find the issue, I would plug it in and let it sit until the noise starts and see if you can physically locate it. Obviously this will require some care on your part but if it finds the issue then you have a chance to fix it. If the transformer is humming you should be able to easily find it just by touching the external insulated portion and feeling the vibration. Pushing on them a little might cause the hum to change if it is a mounting issue. If it not the transformers then touch the heat sinks on each channel and lightly push them a little to see if the hum changes. Good luck and be careful.

Transformer hum is usually caused by core delamination. Unfortunately it will only get worse, if this is the problem. I'm getting a little hum on my 7075 and it sounds like transformer buzz to me. I haven't opened it up yet though to confirm. The only real solution is to replace the transform. Check for loose mechnicals first. The transformer looks expensive.

Note: This assumes the core is laminated iron rather than compressed ferrite. If it's a solid ferrite core then the problem is something else.

Thanks
No hum all day yesterday. Today I am rather busy but I at least intend to disconnect the inputs and outputs. Tomorrow is another day and I will take the cover off....cautiously.

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.