2016~2022 Air in cooling system or in HVAC drain system?

2017 CX9 GT AWD
Just had the engine in my 2017 GT replaced due to the coolant leak issue. After I got it back, I heard what sounds like water circulating under the passenger side dash area when the car first starts up. Didn't do this before the new engine was put in.

Some have suggested in another thread that the air was not purged from cooling system. I contacted the Mazda dealership about that and here's their reply:

The coolant idea does not sound terribly plausible to me, as you would not HEAR a coolant issue, you would feel it, cause you wouldn't get as good of heated air coming out your vents. Mostly likely cause is the HVAC drain system (which is right behind the passenger dash) got clogged and needs to be cleared. It did sit for a while, and we spray down the engine bay with water after replacement to try and get as much of the oil and coolant remnant off as we can. It is possible some extra water got in there and is not draining properly. It is more common on the older models, but not impossible to se on a '17. If you bring it in at some point we can get that checked out for you. Keep an eye out for water in the passenger floorboard, sometimes it overflows into the cabin filter/blower motor area.

I'm probably going to bring it in for further inspection, but interested if anyone has other thoughts on this.
You would absolutely hear air in the engine cooling system with gurgling and swishing of coolant. It is important to get this trapped air out because a pocket of trapped air can cause overheating. The dealership probably thinks you are talking about the climate control "cooling" and not engine cooling system. If you had a clogged EVAP core drain line ( part of the air conditioning system behind the dash) then you would get water on the floor. You'd also get water on the floor if your heater core was leaking as that is behind the dash as well. Also, if they sprayed water to rinse the engine bay then it wouldn't get into in the dash behind the firewall. They stated they sprayed down the engine bay AFTER the replacement? Why would they do that after instead of before? Doesn't make sense. You don't spray down a new engine.

I'd purge the air like I said in the other thread and see what happens. So easy to do.
2014 Ford Explorer Sport, 2009 CX9(Wife's)
I agree you can hear air in cooling system...especially the heater core. I did a drain/fill on my 14 Explorer last fall and when I filled it up and started for the first time, I could hear gurgling in the dash.

I kept adding coolant until it stopped dropping then took it for a drive. I could still hear it when the revs would increase, but it eventually went away after a few more coolant top-offs.
Mazda CX-9 Signature
I have my reserves when it comes to the competency of techs. If you are still having problems and you want to troubleshoot it yourself, I would get a bleed kit and diy. It's quite easy.

Make sure engine completely cold
Install bleed kit on the radiator and add extra coolant
Start car and turn heater on to highest setting
Creep/roll in and out of driveway to disturb any trapped air bubbles
Rev the engine a few times

Hope this helps.
2017 CX9 GT AWD
I did purge the coolant system as was suggested in another thread, but the water sound was still there.

So, took it in to the dealer that replaced the engine. Long story short, the tech couldn't figure out the issue. They did blow out the A/C drain, but no water drained.

I will keep any eye on it (or ear on it) if it gets worse, but disappointed they couldn't fix it. Let you know if something changes.

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