Small Engines (Lawn Mowers, etc.): Craftsman Riding Mower Surging, forward gears, rear wheels

Question
I have a 19hp, 42" cut, Craftsman Riding Mower that is 10 years old.  Winterized and garage-kept, so is in good condition.  Used several times this year without incident.  Started fine yesterday, but made a clanking sound (as if two pieces of metal hitting each other) from the area under the seat (transmission?) when I put it into gear.  Kept surging in all 6 forward gears and also in reverse, but no change in engine power noted.  Clanking sound whenever first put into any gear.  Surging became progressively worse, so I pulled it into the garage after 10 minutes.  No difference noted with blades engaged or not.  Any ideas?  Thanks for any advice.  

Answer
Sounds like you've got an issue with either the drive belt / idler to the transmission or an issue with the pinion gear inside the transmission.  Before you get into the transmission, you should inspect the drive belt (and its path) from the engine to the transmission.  If you don't see any issues, remove the belt from the transmission input pulley.  With the transmission in neutral, rotate the pulley by hand.  It should move freely and you shouldn't feel any "chunking".  If you do, this indicates an issue with broken or stripped teeth on the pinion gear.  If it feels OK, shift the transmission into a gear and put the tractor up on blocks (raise rear wheels off ground).  If you now turn the pulley by hand and feel "chunking", there is a more serious transmission issue.  If you are not sure what you are feeling, have someone familiar with these transmissions check it for you.  Attempting a transmission repair isn't something for a novice but can be accomplished by someone with attention to detail.  Let me know what you find.  Hope this helps.