[Al Skierkiewicz]

Phil,
You have to look at what is causing the noise to be displayed on the scope. If the scope does not have a ground reference either through the power cord or the ground clip on the probe (or both) then everything that the scope is connected to is now 'raised' above ground. This means that everthing between the scope and a real ground is acting like an antenna and inducing noise in that resistance. Old Ohm's Law comes into play, an induced current through a finite resistance develops a voltage across the resistance. You still have a noise source in your neighborhood and I still suspect it is power related due to a transformer or insulator defect. You have just found out why everything is picking up the noise and one method to minimize it. I think you will continue to be plagued by this and will eventually find it. I am reminded of a broadcast problem where a radio station had lots of noise and it was being picked up by everything in the studio. A search with a scope showed excessive noise on the ground buss of the audio console. A bright engineer noticed that the noise was audio modulation. When they detected the noise they found it was the competitors transmit signal. A quick search showed that the competitor was licensed at an address outside of town so no one thought it would be a problem. However, the studios were located outside of town, but the transmitter and antenna were on the building across the street. Normally radio studios keep the drapes closed but when someone opened them for some light, there was this antenna directly across from the studio window. Keep searching and you will find it.
Good news that annother problem has been solved.