A lot of the sounds you hear coming from your car’s engine may originate from the heater core. If the sounds appear to be coming from behind the dashboard, it could well be the heater core – however, given that you’d have to take the dashboard off to access the core, it may be more prudent to get a second opinion from a professional. If the issue is the heater core, get it resolved as quickly as possible or you will risk extensive damage to the car.
If you can hear a gurgling sound coming from the heater core, this usually means that there is air trapped in the coolant that circulates through the core. This can happen for a number of reasons, including a blown head gasket and a leak developing in the heater control valve. You’ll need to diagnose and fix the cause (this may be best done by a mechanic, or you may be able to use K-Seal if the leak is a small one) before you remove the air and replace the coolant.