96 Lumina APV heating issue

My friends vehicle noted above is having some heating issues at idle only cold air comes out from the ducting but when he revs the engine and holds it heat comes out then when he lets off the gas the heat cools down and you can hear "sloshing" liquid moving from behind the dash. I think he may have a clogged heater core but not sure!

Does any one have an idea?
Thanks in advance

FIrst of all, if not already done, check coolant level at radiator...and fill to the top, in addition to normal reserve tank level. Low coolant level by itself can be THE factor, but could also be a consequence of the following.

The symptoms you describe might very well allow you firend to join the very popular club of 3.1/3.4 blown head gaskets, as this is exactly how the heater would behave. In no case a blocked heater core would exhibit those symptoms. But other symptoms would come with that, like overheating, temp gauge going up and down, steaming, and loss of coolant.
Exhaust and air expelled in the cooling circuit by a head gasket leak during compression and explosion strokes inhibits heat transfer and impairs pump action
Another defect, lower intake manifold (LIM) gasket leak, is also a common problem, but it mostly sucks coolant in the cylinders or leaks coolant outside. Since intake manifold pressure is always lower than coolant system pressure, no air is actively (when engine runs) fed to the coolant by an intake gasket leak, but some air can be sucked through the leak when the engine cools down...(instead of sucking coolant from the reserve tank). IN this case though, depending of the location of the leak, coolant could invade the oil pan or leak outside....

So...look for these symptoms to pinpoint the cause, most surely, one of the two.

Low coolant mobility at low RPM is also mentioned for later year montanas and ventures (routing of hoses is not the same as '96) and an auxilary coolant pump is often used as a solution, but "sloshing" or gargles are not part of that problem..