How can I reduce some of the road noise in my 2009 HHR?

2 ways, next time go with a different tire, less aggressive/softer tread. or buy the rolls of dampening material, you install behind each panel, remove the panels, roll the fabric out, and cut it. that works good too. run your hands along the tires in either direction, if half or so is rough they can be "alligatored", which will cause noise,

if the pavement looks grooved, i'm doubting u can do anything. certain areas are grooved for grip/traction.

i have just the plian hhr. i been looking at new ones and the ones carpeted in back are so much quieter than mine. and yes tires makes a big difference

try throwing a blanket down in the back- it will absorb some of the noise instead of just the hard plastic back there.

In most any car you can take out the seats & carpet and put down a layer of self adhesive soundproofing material. Make sure all joints are taped with soundproofing tape, then reinstall carpet & seats. Most of the road noise from below the car will be gone. If you are ambitious you can also remove the door panels and install the same soundproofing inside of the door skin. The soundproofing is heavy, so you may not want to install inside the hatchback, and it will add some definite weight to the car. You'll probably spend $300 to $500 on the sound proofing material depending upon which route you go, but it will be a great investment if you're planning to keep the car and/or drive it regularly on long distances. Hope that helps,