With the radio off, etc., can you ever hear a gurgling noise in the dash? It sounds like an air pocket issue. Several manufacturers had issues like this, sometimes loosening and repositioning the heater hoses can solve it. Try removing the upper heater hose from the core, leave the rest of the system sealed, raise the heater hose above the rest of the cooling system, and fill through the heater hose until you are sure air is purged, reconnect and retest.
