Going to make a long story short.
Got nice new J&M headsets. Before installing in helmet plugged into bike to make sure they worked and got hissing noise, front and rear on both sets of headsets. Tried combinations of headsets and cables too.
Called J&M. They said it was from the bike and was amplifier related. So read lots of threads on our site. Checked out venturers.org too.
I put headsets in helmets anyway (both full face, one Aria and one KBC), thought may as well use them and will do some more research and try fixing in week or so.
We both get on the bike, plug in and no hissing problem. Hmmmmm? Decide I don't care why, let's ride. Later on Colleen gets off, unplugs and I start getting noise, both hissing and engine.
Hmmmmmm? Now I do care. What was the difference? Well with only one headset connected you aren't going to need to use the intercom. Hmmmmmmm?
With only rider plugged in, I found I could control the hissing and engine noise by lowing the IC volume. If I lowered the IC volume to 6 the engine noise went away. If lowered it all the way to 0 the hissing goes away. Ahhhhhhh, very sweet.
Not saying this is the solution for all audio noise cases, we never got any background noise using just the speakers, but when using headsets I am able to control the background noise levels by using the IC volume based on number connected headsets. Works real nice for us.