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My car is a 2011 CTS V sedan, 53k miles with heads,cam, intake, pulleys, E85, FBO, etc. This whole ordeal started last summer when I bought a set of MT ET Street 17 inch 305/45s for the rear American Thrust rims I got with the car. On any launch of more than 1/4 throttle however it sounded and felt as if the tires were scrubbing. So I sold them and put the stock wheels and tires back on. However, the car had the same issue even with the stock wheels back on and the car also started feeling as if it was bottoming out on any kind of bump. So at the mileage and reading of shock failures at 30k, I replaced the rears on Friday hoping that was the entirety of the issue.

Well the car certainly rides better but I still have the same rapid vibration/ noise sensation on even moderate acceleration. It literally feels as if something is vibrating against the frame of the car. The harder you get into the throttle the more prominent it is. Any ideas what this could be? I've asked on FB and someone mentioned engine mounts but the issue only presents itself on acceleration. If its a factor, the car has 3 inch exhaust coming off of longtube headers and the exhaust often catches on the peak of my driveway backing the car out since my lot is a slope. I would gladly take the car to a shop to figure this out since I absolutely don't want to throw parts at the issue but there are no decent speed shops within 2 hours despite the size of the city I'm in. Anybody have any ideas?
 

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Jack car up. Crawl under car with rubber mallet. Start banging on shit. If something rattles or buzzes when you hit it, fix it. Then drive the car to see if the noise went away. Lather, rinse, repeat until there's nothing left to hit with the mallet. If you still have the noise, it's likely a bearing (like CV/Universal) or a gear IMO. It could also be the motor mounts. Yes, under acceleration is where you'd expect there to be a problem. If the mount is totally busted sure, it'll be evident even when the motor isn't running. But it could be "soft" (on it's way to complete failure), and if so, you'd need to load the motor up so it's torqueing against the mount to get the motor to move and make contact with something under the hood.
 

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We’d all be making wild guesses. There are no common problems like you describe.

My guess would be your exhaust is rubbing somewhere. I can get a rod knock sounding rattle if I accelerate just right from a stop. One of my mufflers touches the bumper bracket just right. It took me a while to narrow it down.
 

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I had a metallic clunk when turning after I installed the CS engine mounts but then it would disappear. It ended up being a clearance issue with the steering shaft bolt and primary #5 or 7. With all the NVH going on with a new cam and the stouter mounts, I almost missed it. 😄
 

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I had a vibration on hard acceleration and at speed on the highway..in the 2011 sedan I just bought. Took it to my local shop to get it state inspected. Took the owner for a ride. He noticed it to. Put it on a lift and checked it. Noticed the rear diff was kinda wet....looked closer and saw it was leaking at the axle seal...moved rear wheel and saw carrier bearing in diff moving up and down.... Dealer replacing rear diff with new one as I type this. We checked the drive shaft and everything while under there. Car was spotless underneath except for a little oil around oil filter from recent change...rear axle was a little damp. Had look close to find the issue. Diff was not making any noises.
 
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