I bought my 2004 Monte Carlo SS back in August 2016. It now has 110K miles on it. Last summer the oil mixed into the coolant (but not vice versa) and I had to get the intake manifold gaskets replaced (pretty sure thats what it was). After that, the heat wasn't working once winter came. I tried to bleed out the lines myself, obviously didnt do it right. I got more air into the lines, lost a bunch of coolant, and caused the car to *briefly* overheat (driving on highway, pulled over and had it towed immediately). My mechanic couldn't find any pressure leaks or any signs of a blown head gasket even though thats what he suspected. He said he put some stop leak or sealant stuff in the coolant in case there was a small head gasket leak. He bled the air out, added more coolant, and I was good to go. Had heat all winter.

BUT yesterday I drove 2 hours one way and 2 hours back, with several hours in between. It was 86 F today, hottest day so far this year. When I was almost home I noticed the temp gauge go slightly over half way. It usually only does that when it first heats up, then goes back down right below half. Not a big deal, but it prompted me to check under the hood when I got home, because I also thought I felt some power loss, but I'm paranoid all the time about this car so I'm not sure.

I park in the driveway, leave the car running, pop the hood and everything seemed ok. Coolant and oil were normal. No condensation under the oil cap. No strange noises or rough idling. I shut the car off and a couple minutes later came back to check, and next thing I know im hearing a gurgling/bubbling sound. I open the coolant reservoir and there is coolant and bubbles flowing in. Some steam/gas was escaping out the top of the reservoir as well. The coolant was rising until it reached almost the top of the tank, then stopped. Several minutes later I checked again, it was back down to normal. Took the car for a spin around the block, checked again, no gurgling or bubbles. But I'm paranoid as all hell still.

Heres a link to the video I took of the gurgling. https://vimeo.com/272531974

What could it be? Bad radiator cap? Water pump or radiator is bad, or both (blocked up or something)? Bad head gasket? (but oil and coolant seem normal) Or is this somehow a normal thing where excess hot/boiling coolant in the expansion tank flows back into the reservoir ?? (I don't even know what I'm talking about really on that last part) Or something else? Help / advice would be greatly appreciated. Thanks in advance!