Why is my coolant boiling

[1993Saturn]

I had just done a system drain out and when refilling, I noticed there was a problem when it came shooting back at me in short order when normally it should be just surging up and down at that point of refill still in no hurry of getting hot. So I closed the cap to see what happens. Didn't take long for the coolant to gurgle in the hoses. I've even heard a clicking noise from the thermostat pipe location like a toy clicker cricket they used for signalling in WW2. For a long time after shutoff, it continued to rumble. All along it was also shoving out the cap onto the ground and filled up the reservoir.
New Murray 3199 Thermostat, Cap, hoses.
What ya suppose?

 

[410Fortune]

if your water/coolant mixture is off it will boil too early
If your cap pressure is off it will boil

Also blown head gaskets can cause bubbling in the overflow/radiator, air pressure from cylinder heads is transferred to cooling system, its not actually boiling from heat, its air escaping the engine

Did you replace the thermostat and cap trying to solve this same issue?

Here is a good read:
What Does It Mean if Car Coolant is Bubbling?

In my experience it is almost always blown head gaskets

 

[1993Saturn]

That ain't good news. I can't understand why the gaskets could be leaking. I put the exhaust manifolds on the heads first, dropped them into place on top of the exhaust pipe and torqued them all down by the prescribed sequence with Permatex thread sealant on the bolts. With 190+ torque on those bolts, how could it possible leak? And compression air going into the coolant wouldn't that also mean the pressure would go the other way and the cylinders would suck in coolant giving me white smoke and coolant smell out the tailpipe? I replaced the thermostat because it had been a while and I was there anyway during the process. I replaced the cap because the gasket was cracked.

 

[LG57]

Air in the system? Let it cool and refill it.

 

[1993Saturn]

I was writing a reply to another thread " Goofy engine Temp issues" about the famous Explorer trapped air in the coolant problem causing soaring temp readings or thumping in the heater core and I got ta thinkin' (dangerous!) When I pulled off the heads, I had syphoned all the coolant out of the front of the block yet when I pulled the heads, coolant was trapped in the heads and the chambers in the block next to the top of the deck. It left quite a mess in cylinders and off onto floor. SO, if the coolant was trapped up there, then wouldn't it also hold true that the air gets trapped up in the heads after being emptied of coolant and won't let in the coolant fill those voids? This may seem true when I listened to the system this morning after driving. The bubbling noise appeared to be emanating from the heads.
Now, in what fashion could the solution be to filling up those voids and silencing those tea kettles from boiling and ruining my day?

 

[1993Saturn]

As I asked earlier - " And compression air going into the coolant wouldn't that also mean the pressure would go the other way and the cylinders would suck in coolant giving me white smoke and coolant smell out the tailpipe?" Is there anyone that can explain how that works? I'm baffled. Thanks.
Now, cooling down and refilling, it soon comes up to super heat and coolant dumps out on the ground from under the cap. I just replaced it with another new one to see if it was a fluke. If I were to flush the system, will that fix it. I have a water pump that seems to be making a metallic noise that is not like any water pump I have heard before almost like a snapping. What if the new thermostat isn't opening up and trapping air.