1998 Chevy Camaro real loud clanking

I've been hearing a loud continuous knocking or clanking from under the hood. The noise is only in the low rpm range under 1000rpm's. Once the engine is above 1200rpm's the noise is gone. I've had a slow coolent leak for sometime and have gone empty a few times. I was wondering if it could be a water pump bearing faliure. I've had engine knocks before and also failed rod bearings. This sounds nothing like that due to it is only at very low engine speed.