This is a different issue than I've had with older GM cars, which always did it after over-torquing and/or water on hot rotors.

I bought this 2011 LT1 used with about 45,000 miles; was great but had bad tires. Put new tires (Michelin Primacy MXV4-H rated) on, still no problems. Had them rotated at about 53,000. I watched rotation, 85 lb-ft torq stix, last partial turn to 100 via torque wrench. I rechecked 100 miles later, one single nut moved a TINY bit, so I think all was ok.

A week or so later the problem started. I can drive and make gentle or moderate stops for 10 or 15 minutes. Then subsequent brake applications have various amounts of vibration, whether very light or digressive. If I park it for a while (several hours), it's good at start again.

I pulled the wheel and ran an indicator on cold rotors ... really good, 0.0015 total indicator runout. Can judder come and go? How can it be diagnosed?

Thanks!

 

Yes it can come and go with the heat expansion of the rotor. When you used the dial indicator did you check the runout on the back side of the rotor?
Here in the salt belt we see the back side of the rotor rust and become out of round.
The rotors look ok at first look but when you try to resurface the rotor and make the first cut it opens up rust pockets just under the surface.
When this happens all you can do is replace them.

 

Judder can come and go as the rotors heat up and cool down again in normal driving. If the rotors are close to "discard" thickness they're more prone to juddering, and a few moderate stops can build a surprising amount of heat in the brakes, even though you wouldn't expect it too.

Your runout is fairly nominal, but you also need to consider the deposition of friction material from the pads to the rotors, which can be caused by front brakes being overworked.

HHR's are "blessed" with self adjusters on the rear drum brakes that are less than effective. Those of us here who have dealt with brake judder in the past have a maintenance mantra that involves manually adjusting the rear brakes every 5,000 miles or at every oil change.

Give this thread a read on how to adjust the rear brakes, once you do it once or twice it's a matter of minutes to do it on a regular basis.

https://www.chevyhhr.net/forums/brakes-%7C-suspension-%7C-shocks-%7C-struts-24/proper-rear-drum-brake-adjustment-procedure-36754/

Plus you want to make sure that your calipers are free on their slides, and check the thickness of the rotors. Unless GM has changed the specs, discard thickness on the rotors is 23mm.

 

Bearings.

 

A distinct possibility Don, but he hasn't mentioned the biggest symptom of failing hubs, that lovely and sometimes too familiar roaring hum from one front or rear corner.

But definitely something to add to his "check this too" list.