Gurgling/Bubbling after Major Service

Hi everyone, I hope everybody is doing well. I was wondering if I can get some insight on an issue I've been having. (2015 125k mi)

I attempted to search around the forum, but couldn't find an adequate solution: I seem to have a gurgling/bubbling sound issue sometimes when driving. I would like to guess this is air in coolant loop.

Given that:
- No warning lights, no DTC codes
- Oil and Coolant were both drained and refilled. I followed the procedure by CarCarNut. Left the maintanence mode with heater on HI for 50 minutes to attempt air purge. Max engine temp during this was around 75C/167F
- Drained Oil and Coolant was without any signficiant impurities or globules. Homogeneous liquid as expected. I saw a post here about this being a symptom of using gasket sealants(IIRC), and using 50/50 vinegar water to check.
- Recently cleaned out EGR cooler, pipe, and replaced the EGR valve (preemptive service, no issue). Replaced all gaskets as well.
- Intake Manifold cleaned. It already appeared unusually clean for a car with 125k mi with fully cleaned out ports including the problematic smaller ones, which I am curious if this is indicative of previous owner having engine rattling issue. Preemptive service.
- PCV Valve replaced and cleaned throttle body. Both preemptive service.
- Other minor part replacements like spark plugs. Preemptive.
- My driving habits are usually ~2x30mi drives every other week. I am a student in Southern US.

Observations:
- Gurgling/Bubbling noise when driving, I am by default assuming this is trapped air bubbles in the coolant loop, or something with heater core. I believe this correlates with engine load as expected.
-After gurgling/bubbling observation, I put it in maintanence mode + heater and ran it for about an hour. No gurgling/bubbling appeared during it. Max temp around 75C/167F ish again according to Hybrid Assistant.
- Coolant and Oil levels are steady. I've been checking the levels since last month religiously which is when I performed the oil + coolant drain & refill as per CarCareNut.
- It happens at minimum when the windshield defogger is on. I am unsure if this would happen without the windshield defogger, but I would think that this is equivalent to turning on the heater. I have driven a few times since the coolant work, but it could be that I have just missed it due to music playing in the background.

Current Plans
- I plan on doing the usual 30mi trip back to school tomorrow, so I will try to observe the engine temperature during operation. I have not seen the overheat lights come on yet.
- If it is at the end of the drive, I will try to record it and upload it.

Current Impressions
- From searching around the forum, I am guessing that this is likely due to air trapped in coolant loop, or something to do with the heater core.
- I am unsure how I can check the performances of thermostat and the water pump, but I would like to investigate into these parts as well.


I would like to ask for some opinions or insight on what could be the reason for the gurgling noise I've been hearing given these conditions? Could this be possibly caused by me not driving long enough to fully open and clear out past the thermostat? I believe my observed maximum engine temperature of 75C/167F during maintanence mode is quite inadequate to even open up the thermostat a little. In such case the coolant loop past the thermostat is practically locked off under maintanence mode. I'm wondering if this just a somewhat common thing after coolant change as a temporary symptom until it stabilizes.

Thank you so much guys. I'm a new driver with Prius as first car, and I've been learning to make sure it lasts as much as possible.

 

Thank you,
The engine coolant level is holding very steady, drove extra 60 miles today, and seems to hold quite steadily having checked it every time before and after it's driven. I've been a little obsessed with that, haha.

I think it is quite likely that gurgling is related to refrigerant. Since the post was made, I was not able to hear any of this noise without the defogger on, which I think could explain the issue. I did not smell any coolant (sweet smell) nor observed engine over heating (usually between 180-200, which seems nominal after doing some searching)

If it is just the refrigerant, then I would be a little relieved (unless it indicates some additional issue) Just was quite concerned if it would mean coolant loop issue prior to engine overheating.

I bought it in November, but I did a full major service as listed prior to driving around consistently. So effectively around a month and half. I would say I probably have driven around 400 mi~ ish since then. Also thank you for commenting Mendel! I lurked around priuschat for all technical discussions before purchasing and feel a little star struck here.

Trying to do my best to address everything if I can.

Pixel 6a ?