It's almost unpreventable, in that the low hoodlines of today's cars can mean that the heater core may be one of the highest points in the cooling system. Overnight, coolant will settle and system air will collect in the core.

If your coolant level is OK, then you likely have no problem. Ecotec engines are notorious for forming air pockets in their cooling and heating systems that, while not detrimental to engine operation, can lead to unique behavior like this.

 

Our other vehicle is a '97 Grand Caravan, and it exhibited this problem for quite a while, then I noticed water leaking into the interior.

Turned out that there is a fairly big pan under the cowl vents that catches water. On the Caravan it probably holds a liter of water or more. There are two drain holes in this pan, with a hose attached to each to carry the water down past the engine block. The pan was not emptying because these hoses were plugged up with dead leaf debris. I ran a stiff cable through them and then blew them out with compressed air, and the problem is gone.

I can't imagine being able to hear fuel sloshing in any vehicle as well insulated as the HHR. I drove plain vanilla pickup trucks for years, and even with them you couldn't hear sloshing fuel.

The theories about coolant might be worth examining. I had a Suburban whose cooling system made a sloshing noise. Because it had a 454 engine and rear heater, the system held over 7 gallons of anti-freeze, and it was occasionally possible to have an air pocket form in the system. Eventually we found antifreeze leaking at the intake manifold gasket, and producing a chronically low coolant level. (Wish that was my worst problem with that truck. I totaled the 'Burb in October and got a broken neck in the process. But that's how I ended up owning the HHR.)

 

Just had my '08 in for this a couple of months ago. Also had water leak - just a few drops - now and then. They found the drains in the cowl plugged. Cleaned them out and replaced the blower motor they said was water damaged. No more sloshing. Of course it is winter here and any water would probably be frozen anyway

 

Hi. I am curious if this fixed your issue? I have an 09 now for a couple of months & I have that noise in the morning when I start to move, too. It does it sometimes throughout the day as well, on acceleration. Thought I was just "nick picking". I have looked under the hood to see if something was just loose and rolling around in there! If this fixed it, let me know what you did!

 

My 06' did this from day one and NEVER had low coolant. Bill had the best explanation, the heater core is higher than the resivoir

 

Have been hearing this for some time, usually first thing in the morning as well. Took it to our dealership and they want $1300 to tear out the dashboard and drain the a/c condensate line. Told them to forget it, I will live with it. Does this sound like the solution as I don't want to spend $1300 on this!

 

Get somebody to check your coolant level. Both the tank and the pressure cap. Dollars to donuts you have never checked it and it is low.

Unless you have a 2006 with a surge tank, then you can see it easily.
The coolant should be changed (with DexCool) EVERY 150,000 miles OR 5 years. The time limit is actually more important than the mileage.