The Attic Air Handler of our HVAC First Co. unit makes a gurgling noise when the AC is on. This does not happen when the Heat is on. This is an oil / hot water / cool water system. Any ideas on what this is?

Help is needed to answer this question. There are three possibilities as to what this could be causing this noise. The first thing to check is the condensate drain. If the "P" trap is not deep enough, the gurgling is the water being sucked back into the unit instead of draining properly. This will require a re-piping of the drain. If you only hear the noise when the unit first comes on or after it shuts off, that is normal. The refrigerant will gurgle until the system pressures equalize. If the noise is coming from the refrigerant lines during normal operation, the system charge may be low on refrigerant. Please re-send when you can isolate the noise, and I can tell you how to fix it.

Most air handlers are pull through coils. If there is not enough return ductwork, it doesn't matter how deep the trap is. Dirty filters will also exasperate the problem. Problem is trap or static pressure, hope this helps.