225 Mercury 4 stroke rumbling noise and low idle

[surfsidewood]

I have a 2003 225 Mercury 4 stroke that seems to have a low idle problem. The motor runs fine at all speeds except when idling in gear it makes a rumbling noise and runs rough like it wants to stall. If I bump the throttle up a little bit it smooths right out. The rumbling noise kind of reminds me of having a bad ujoint on my truck. when I run the motor on muffs in idles low for a minute then speeds up and idles steady until I shut it off. Any help would be greatly appreciated.