Is it common for the baffles inside the Resonator to become detached? Mine has an annoying rattle that seems to be internal. If you hold it still while it's running, the rattle will stop.

 

In your 2009 2.2 litre exhaust it’s pretty common to have that rattle. At 15 years old, your HHR deserves a new resonator, companies like Walker have replacements available , here’s an example

https://www.partsgeek.com/dbzccsb-ch...-assembly.html

 

There’s a heat shield on the transmission mount to protect it from the heat from the nearby cat. It can get rotated out of position and rattle on the exhaust system. Just something to check.

 

A roll of heavy but pliable mechanics wire and pliers wrap around heat shield and twist tight.
Many times its the bolts that mate the halves that rust away. or another heat shield nearby.

Internal sounds like rocks in a can when you tap on it.
Do all this at ambient temperature and work gloves of course.

 

This heat shield,

Part #1

 

Or this heat shield? Part #6.


I don’t think it’s either , just the inards of the resonator

 

After looking at the photos in more detail, I realized that I referred to the Muffler as the Resonator. So far none of the heat shields appear to be loose. While idling and rattling, I will pull down on the tail pipe and the rattling will stop. The heat shield on the muffler is not loose and all the exhaust hangers seem in tact. So it seems an internal part is loose inside the Muffler.

 

There is a heat shield were the exhaust pipe goes over the Axle , mine was loose and broke off easily. No more rattle.

 

Oh, The muffler! Yeah, that’s all rotted out and rattling around, and probably not breathing very freely. Replace it.

The 2.2 does benefit from an exhaust pipe for a 2.4 with its longer resonator and larger diameter. If it was my car, I would get an exhaust pipe/resonator and a muffler for a 2.4. Bolts right in.

 

Agreed, I did that on my 2007 2.2 LS much improvement in breathing, I also replaced the catalytic converter with a mandrel bent 2 1:4 inch stainless pipe.