Yamaha Motors - Prop spins while in neutral

Question for our resident mechanics or knowledgeable Yamaha guys or gals on the site, I have an question. I own a 2001 Grady White Islander with twin 200 Yamaha HPDI outboards. Bought the boat used in 2010, motors had `300 hours on them, counter rotating. The port motor since we purchased the boat has turbulence at idle, starboard motor does not. While in neutral and giving it no throttle, the prop on the port motor spins causing turbulence. You can actually see the prop spinning even though the gauges are showing zero RPM's. It doesn't appear to cause any performance issues but I'm not sure why the prop would spins when in neutral. It only happens with the port motor, not both.

Anyone have any guesses what might be causing this, never seen it on any other boats I've owned. As I said, it doesn't seem to effect performance but would like to know what's causing it and make sure it doesn't effect performance at a later date.

Appreciate anyone who might have some insight.

Thanks!