When I rev the engine I can hear water rushing through the heater core. Any ideas why this happens? I have a 2001 ram 5.2L.

 

you have air in the system. either because you have a leak, where coolant leaks out and air leaks in, or because you've opened up the system and haven't yet purged out the air.

if no leaks, make sure the overflow is full, and it will generally push it out on its own. if not, you can lift the front and run it awhile with the cap off.

if your waterpump is leaking, the air will never purge.