I have a 2008 LS with only 35,431 miles. I had the same problem. The right side hub was the worst of the two (I replaced bot sides). It rolls down the road rumble free now. I also replaced the front brakes and rotors while I had it all torn down. Holding the hub in my hand an turning it you could feel and hear the grinding.

It seems that a lot of HHRs have the "Brake Shakes" when applied. Mine has had them almost from day one. One of the reasons I did the brakes and rotor was to get rid of that and it worked. Nice smooth stops now.