FWIW .... I've had just the opposite issue.

I've owned 10 different HHR's both 2.2L and 2,4L auto trans.
I would say that at least 6 of them including the one I am currently driving has had a light knock when Cold and lasting for about 2 miles then gone.
I do not hear it when cold idling but as soon as you give it some gas and load it in gear it's there.
The first one I noticed was a 2.2 and I sold the car because of it.
All the HHR's had approx. 20 - 28,000 miles on them when this was going on.
I have been running Mobil 1 in the last 3 but just switched to Pennzoil Premium Full Synthetic in this one to see if it made any difference. I change every 3000 miles.
I does seem to be just a tad better but it also depends on how cold it is when I start out so it is a variable test each time.
The point is, it seems to be almost normal to some degree.
In the warmer part of the year it is not an issue.

Silverfox

 

Hello , I just recently did the timing/balance chains and I am getting a knocking noise from what sounds like the top of the valve cover. It's a 2007 hhr 2.4l . It's got a new belt, tensioner was good, and replaced the mount that is on that side of the engine. Any ideas what it could be. The noise is only audible when I hit the accelerator. At idle you cant tell it's there.

ps. I did this on the car there is plenty of room. Just need to be able to jack the engine up and down for different bolts.

 

Best to remove the valve cover and visually inspect, look for a broken front guide, check the timing chain tensioner , is the timing chain loose ?



This was loose enough to cause problems, the chain was slapping that guide, the front guide upper bolt was also damaged.

 

After a little more investigation, turns out I might have a bad rod bearing. All tensioners are good.
I am going to drop the oil pan tomorrow, or the next few days, and check for any play in the rods.