2007 250 HPDI prop spins in neutral while idling on hose

I have a friend with a 2007 250 HPDI. When he hooks the motor up on a water hose and lets it idle in neutral, the prop spins slowly...then after a few seconds, will start to get faster and faster. A mechanic friend of ours said this is normal, and is caused by the exhaust. Does this sound right??

Yep that is normal as long as it does not get to turning very fast like you would have it in gear at an idle.

normal