Some times they might spin with the engine running only B/C of vibration and that is REALLY slow and you can hold it with your hand [SLOW] and it stops and some times with a SS prop on it will do it a little more while on the muffes. Now if you hear any clicking then it would be trying to go in to gear. And if it still spins in the water while in neutral then you have problem with the cabel not aujusted right or it is streched out or you have a problem with the shifting spool inside the lower unit of the drive. Try playing/ moving the cable mounted to the engine and running down behind the engine a little to see if you can get it to stop moving.is it possible the exhaust moves the prop around ? [kind of] also as long as it is idling you can put it in gear to do some testing just try not to keep doing it all the time and you will be fine.
