[Dngrsone]

Sounds like you may have a loose connecting rod (pun not intended, much). Connecting rods take the explosive force pushing teh pistons and apply it to the crankshaft. If the bolts holding the connecting rod to the crank work loose, then you will have a clanking-type sound. You will also have a limited amount of time before one part or another breaks and does some catastrophic damage to your engine.

At any rate, it sounds like a good reason to stop running the vehicle and getting someone to take a good look at it.