You follow the sounds of skittering and scratching to a spot at the edge of the Tavern's cellar, where the source of the plague of rats immediately becomes apparent:
Somebody left the rat faucet on. How careless!
|Turn off the faucet|
You close the valve. The torrent of rats slows to a trickle of rats, then finally to no rats at all.
You should probably go tell Bart you've fixed his rat problem.
Occurs at The Tavern Cellar.