After car has been driven you can hear cooling system boiling

Sometimes a smaller radiator can be more efficient than a larger one but you describe a situation where the coolant is boiling which is very bad as it can soon cause serious engine damage - if it hasn't already.
Clearly the vehicle should not be driven until the radiator can be replaced with either one identical to the one the manufacturer fitted or a genuine upgrade - providing increased flow, capacity and surface area...

While this is being done the cooling system should be flushed and the thermostat replaced before refilling with coolant and checking for leaks. Only then should it be driven and assessed for any engine damage caused by the previous "mechanic's" efforts to make a fast buck...

How hot does it get in degrees F? Does the cooling fan turn on? Is the coolant boiling over?
As far as your temp gage, maybe you have air in the system? I would think as far as you have driven, if you didn't bleed the system, it should have worked it's way out?
You could have combustion gasses getting into the coolant because of gasket issue?

Low coolant, bad thermostate, bad cooling fan. Blown headgasket but unlikely unless the car was driven hot.

Check thermostat. The boiling sound your hearing could be a restriction in the heater core under the dash.

What you are hearing may be normal.
There may be air pockets in the cooling system and you are hearing the coolant moving around in the system. Kind of like a coffee pot.
As the thermostat opens at 195 degrees the coolant expands and contracts.
Make sure the system is full of coolant.

no coolant is low there is a air bleed procedure that must be done to fill the cooling system properly

Your coolant level is low, and you hear the coolant taking up the vacant areas....bubbling...due to low coolant.

Do you have a coolant reservoir? Does the coolant level rise as the engine warms up and lowers as the engine cools off? If so, the engine cooling system is sealed. If the system isn't sealed, when it is cooling, it can't draw coolant from the reservoir. Does the radiator have a radiator cap, with cold engine, remove the cap, can you see coolant level in radiator?
How hot does engine get in degrees F? There is your thermostat and radiator cap. Water boils at 212 degrees f atmospheric pressure. With a 16 lb radiator cap, it should raise the boiling point about 32 degrees. The boiling point should be around 250 degrees F. w/o any pressure, I would suspect it would boil over in a hurry.

Look for low coolant and be sure to have heater on high when checking coolant level...

Hotter than 1/2 way is not necessarily a problem, do the fans cycle on and off?

You are most likely slightly low on coolant. I also suggest that you pressure test the system as well as the cap. Leaks allow coolant to escape and your vehicle is fussy about perfect level...

REMEMBER that when you release the pressure on a cooling system, non boiling coolant can now boil! Many people get seriously hurt... This is NOT indicative of a problem, but rather NORMAL physics, just crank it in to your proceedure...

sounds like the thermostat may be sticking closed. i would check to see if the thermostat is opening or not