This definitely will not be in the torque converter, it is most likely going to be either the coolant like you suspected or the refrigerant in the AC system. In either situation it is an indication of potentially being low and needing service. There is a third possibility as well I will address in a moment.
The first thing you should do is make sure the noise still happens when the AC is turned off. This is a very easy check that will verify if there is a refrigerant problem. If the noise goes away when the AC is turned off, then you know there is a likely a low refrigerant charge condition. It is worth noting that the refrigerant can make this noise under certain conditions while it is still pretty close to proper charge, but generally speaking when you hear this noise it is 10% or more discharged.
If the noise is still present with the AC off then we would know it is not refrigerant related. At that point it is most probable that there is air in the cooling system. When air is introduced to the cooling system (either via leak or evaporation) it will create a rushing sound heard through the front of the vehicle and/or dash as the air pockets push the coolant through the tubes to the heater core etc.
When this happens you will want to get it addressed as soon as possible; that would mean checking for external leaks in the cooling system, pressure testing the system, and if nothing is found refilling and purging the air from the system. Left unaddressed, you do risk potential cooling related issues.
The last thing to consider, and this is only typical on certain models, is if you have AC condensation trapped in the dash from a plugged drain. This is again only typical on certain models (Endeavors, earlier Galant/Eclipses, etc), but is luckily pretty easy to check. Driving at low speed in a parking lot or other open space, steer the car hard in one direction or the other and see if the noise is reproduced that way. This is using inertia to move the water around if you have a clog. Likewise if you steer real hard to the left and there is a good amount in there you will hear it hit the fan as well which will make a unique noise.
Based on your description with it being infrequent I would say it is less likely to be that (AC clog) and most likely refrigerant level or coolant. Both are very common complaints across most of our product line.