I just bought a boat recently with a 2001 40 HP Merc.  When I spin the prop by hand, it makes a ratcheting sound.  I just rode on my buddies boat and it's silent (but also brand new).  I drained the lower unit oil and put in fresh and didn't see any abnormalties.  There's no signs of leaking after running it about 10 minutes last week.  Do I have a problem or is it just not as tight as a new motor and it's normal?  The prop shaft has about 1/16" play (fore-to-aft).  Thanks.

My 1988 25 Merc does the same thing, and has for years, this is especially true if I shut the motor off while still in gear and coast into a spot. It is a quick reminder to slow down, take it out of gear and then go to the front deck and fish.

That is perfectly normal.  Mercury's have a ratcheting type drive system so the prop will free turn when you let off the gas.  This prevent's a problem called decel knock common with motors like the OMC/BRP's when you back of the throttle quickly.  The clicking sound causes no harm.  If you put the motor in gear and spin the prop by hand, you will not be able to turn it in one direction but will spin somewhat freely in the other, making that same clicking sound while doing so.

Thanks so much!  I feel better now.

I should add that if the motor is in neutral the prop should free spin with no sound, the ratcheting sound should only happen while in gear.

Yep,

I went for a 2+ hour shakedown on the Tennessee River on my new (used) boat and it ran great.  I checked the prop afterwards and it spins easily w/o a sound.  The shifter must have been off neutral.