While flowing down a stream of water in a minecart you hear the minecart moving across rail sounds.
Steps to recreate.
Set up a rail to go into a water stream preferably down a hill. Get in your minecart then ride until your in the water. Once the minecart you are riding is flowing in the water down the hill you should still hear the minecart rail sounds.
The sound is not based on if the minecart is on a rail or not it is just the generic moving sound for the minecart