Hi, newbie here! I just got a used 2007 HHR and I love it. But it's making a low, pulsating humming sound when I hit highway speeds (55mph and up). Doesn't seem to happen at lower speeds nor does it get worse/louder when turning the wheel. Ride feels smooth except for a pulsing vibration. Alignment seems ok. Doesn't get louder when brakes applied. When I bought it from a private seller, they had the front passenger wheel bearing fixed and new brakes put on before I paid as these things were positively necessary to fix prior to purchase. Part of me wonders if the brakes were done wrong or are rubbing, I can't hear it when I open the window, but only inside the car...otherwise it seems to be running fine. The tires in the rear are a bit worn on the insides but not horribly. I'm at a loss. Thanks for any direction you could give!

 

Run your hand lightly over the 2 rear tires in a fore and aft direction to see if there is any cupping on the face of the tread.
That can cause noise and vibration issues if it is bad enough.

SF

 

Welcome to the site, it could be the cupping of the rear tire treads, it could also be a rear hub, the other front hub, or simply the lug nuts were over torqued. There is also the plausible case of the axle nut was put on with an air gun and over or under torqued.