Sounds like water running on Pass. side

Every time you push the gas, wether driving or just in park reving the engine you can hear what sounds like water running just behind the dash. I am pretty sure it's the heater core I'm hearing, also there is only slight heat in the front and none in the rear, pretty sure there are 2 seperate heater cores. How hard is the front and back to replace, or is it possibly something else??? Thanks!

There may be an airpocket in the heater core or one of the lines. Is the cooling system and overflow tank filled to spec?

Could be, it's filled right but ya def. could be an air pocket. It has DEXCOOL in it which I hate so I'm gonna drain it, fill it with water, run, drain, and fill with water again to get it all out, as well as do a flush, then fill with the green Antifreeze and see if that doesn't help, thanks for the info!