Thanks for the advice Cam. I don't think it's light pollution as there is only one light dome and it is to the southeast while I was looking north. It tends to appear on the side of the sky closest to the horizon but that occurs all the way around the sky. Somehow I always image things low in the sky so that probably doesn't help.
I like to play around with settings a bit - I use ISO 200, the cameras minimum, for the moon and planets and usually push it up to between 800 and 3200 for DSO's. One of the reasons I was using it was the camera is on a simple non-tracking tripod and at 15 seconds the stars started to trail. For the next target that night, Orion, I did one at 400, one at 800 and a few at 6400 just to see which worked best. The 400 and 800 produced much darker backgrounds but were about 2 magnitudes short of 6400. The 800 looked best overall but silly me used 6400 anyway
