Engine MCGHS - Won't turn over

Ran the mower for 3 hours all went well. Shut it off and the next day opened the "lid", lifted the seat, and sprayed all the leaves off the engine with a hose. Let it dry. Several hours later went to start the engine all that happened is a metallic click sound. Sounded like the starter was trying to kick in but all that was heard was a click sound. According to the owner, it was they sound they hear when the battery is to low to turn the engine over. Put a battery charger on the unit for 24 hours and no improvement. Removed the battery a week later, cells are full, and the voltage is 12.32 V, battery was new in 2013. Mower does not get a lot of use.

Any thoughts as to how to get the engine to turn over, or fix the metallic click sound.

Thank you.