Thanks.Without reading through the entire thread, this looks like the lack of a ground. If you are using differential input to the scope, the source still has to have some kind of a ground reference or the noise will get though.
Tie the safety ground and the neutral together and connect the scope probe ground clip to that point. Then connect the scope probe tip the the hot output. Look at it singled ended. Worst that could happen is that you could blow a fuse/pop a circuit breaker in the generator/inverter/wall outlet.
You and reloadRon both suggest the same thing so I'm sure that's it.
Your patience is appreciated.