Over the years I’ve owned 4 Panels and they have all suffered from the same exact knocking noise that comes from the underneath rear end, my current Panel only has 51,000 miles
It only happens when going over speed bumps, the noise doesn’t happen when accelerating or normal driving.
I’ve have just always ignored it figuring it was a defect that’s common with all HHR’s since all of mine have had the exact same noise.
It almost sounds like loose shocks but that’s just a wild guess.
Any input would be greatly appreciated.
Thanks, Davyo

 

Check for broken spring

 

Either a broken spring or loose shock bolts.

 

4 different HHR's with Broken Spring ??
Them's Pretty Big Odds.........

Somewhere way back I read a post on this site that the HHR owner had a noise in the Rear that he just could not find.
In desperation when he was replacing the rear shocks he noticed that the bolts were threaded over most of the length,
He just took a chance and found some new tempered bolts with a much shorter thread length and put them in.
That was the end of the noise.
He does not indicate if it is on one side or not.

Silverfox

 

Got same in panel. Try to look at spring insulator, upper one. There should be bumper and thin rubber ring. Also check muffler hangers.

 

Very interesting info about the bolt threads, I shall definitely mention that to my mechanic.
On a related note, as I recall, I think all 4 of my Panels have been 2009’s by coincidence, I wonder if my knocking issue was only on 09 years.
Thanks Silver Fox for the info on the bolts.

And thanks everyone else for their feedback so far as well.
Davyo

 

Thanks, great suggestions.

And thanks everyone else as well for their suggestions.
‘Davyo

 

The shocks only last 50,000 miles. The spring insulators are notorious for wearing out. The swing arm bushings could be bad after, at least, 9 years. Is the spare tire and jack stowed properly? Is there loose stuff under or around the spare tire?

 

I would take a look at the rear bumper cover. The way they are designed, the cover just sits on back inner bumper. Not bolted down tight.
Hit a speed bump or similar bump and the cover actually "lifts" a little, then bounces back down which makes a thump noise.
I shoved some foam rubber scrap pieces around the cover on my SS to tighten it up. Eliminated most of the noise.

You can reach down under the bumper cover with both hands and pull up on it. If it has any movement, then that could be an area you would need to tighten up.

 

Same shock bolts on all years and models, more likely they aren’t tight.