There's a simple way to tell if it's from your outboard.
Get up to scanning speed, then shut off the outboard.
If the interference goes away, you know it was that.
If your sonar is hooked to the battery the outboard is
charging, you will most likely get interference.
Get yourself a separate battery, only for the sonar.
It doesn't have to be a big battery.
From what I've heard, the slowest speed is the best scanning
speed. You could be getting some water turbulence/water aeration across
your hull at 3.9mph.