Chevy Sonic 2014 Sloshing Sound

[meho118]

Hey Guys I'm new here.
I have a weird problem with my chevy sonic 2014 1.6L no turbo.
I hear sloshing sound in the passenger side everytime I rev the engine about 1.5 to 3 RPM, I can't stand this sound anymore I went to the stealership, he flushed the coolant system with new fresh coolant , but I still have this sloshing sound , now he said he doesn't know what the problem is, might be the thermostat or the heater core..I'm very confused, any ideas ? I attached a short video of this issue.
Thanks..!!!

 

[SkyDog]

It's common to hear a sloshing sound from the passenger side under the dash when you've got air and coolant both flowing through the heater core. It's common if you're low on coolant, or if you've recently refilled coolant and the car hasn't bled all the air from the system yet.

Unlike some cars, the Sonic will self-purge air from the coolant system. You don't have to manually bleed it. As air is displaced in the system, more coolant may be drawn in from the expansion tank where you add coolant, so check the coolant level for the next few days after a refill and top it off if need be. (Just to be clear, "top it off" means to add coolant to the expansion tank fill line -- not to fill the tank to the top.)

In some cars (not sure about the Sonic), you can hear a similar sloshing sound from the same vicinity if the AC drain is clogged. Water will back up in the drain line until it's in your HVAC blower housing and you'll hear that slosh around. It wouldn't vary with engine RPM, though, so we can be pretty sure that's not your problem. I'm pretty sure you're hearing coolant & air flow through your heater core.

There's another discussion on the same sound here:

running water sound from from passenger floor area

in the mornings when i leave for work i take off and hear a water moving around . i assumed that it was from morning dew or moisture from the night and the water was just draining from the seams in the body. i figured i would only hear the noise once until i leave for work again the next day...

www.sonicownersforum.com