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Discussion Starter · #1 · (Edited)
Last week I bought a new 2018 GT that had about 20 miles on it at that time. I've noticed a wump, wump, wump helicopter-type noise when going 30+ mph on some newly paved, smooth asphalt roads near my house. It's very, very subtle when going straight, but is greatly exacerbated by swerving (especially swerving to the right). The noise increases in frequency with speed and occurs whether in AWD or RWD mode. You can also feel a slight pulsation in the floorboard and steering wheel when it happens.

I suspected it could be flat spots on the tires from sitting on the dealer's lot since last Fall, but now that I've put 200 miles on it I would think the tires would be back in shape (maybe not). Regardless, I swapped the tires from front to rear and torqued them down evenly to spec, but that did not make a difference.

Is this behavior common to these cars?
Could it be a component of the AWD system?
Could it be the electronic stability control?
Maybe a bad wheel bearing ?
The tires are Michelin Primacy MXM4, maybe this is a characteristic of them?

I thought I would post here to see if this is something typical or obvious before taking it in for service already. Any thoughts greatly appreciated.

EDIT 10:57 pm
After originally posting this, I decided to jack it up and record both front wheels being spun by hand. Based on these two recordings, I would say that at the very least, the tires are out-of-round. I also wonder why the rotors aren't spinning smoothly through the pads.

Front-Left Wheel

Front-Right Wheel
Note that you can audibly hear the bearings clicking in right wheel, whereas they were silent in the left wheel. I'm not sure which is normal (or if both are normal).

What do you think...tires or the clicky bearings as the culprit?
 

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That amount of runout you see is exactly what you would notice on a tire balance machine. New tires need settle in time. And yes, sitting on the lot for months can cause flat spots. My car has been in storage since November. When I take it out next month, I "know" it will have a little tire tramp for the first few miles I drive it.

-John
 

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As far as the shoop shoop sound of the rotor, that is normal. In the early days of disc brakes, the natural runout of the rotor was what was relied upon to push the pads back from the rotor after the foot was removed from the brake pedal. In the interest of fuel economy, back in 1983, GM developed the "low drag" disc brake. It was a re-invented piston seal that would have a few thousand's of return influence on the caliper piston so as to get the pad completely off the rotor within reason. Other makers followed suit as the years went on. Someplace there is an animation on you tube but, I haven't been able to post any links or photos due to the site issues going on.

-John
 

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I had the same issue with a car I bought new a few years back. It got more pronounced after 500, 1000 and 1,500 Miles. After bitching up a storm dealer put 2 new front tires on. Magically noise was gone but started again after 500 Miles. I was furious. I demanded that the measure the rims for roundness and uniform width. Told me not possible on a new car unless I hit something. F that. I had to run this way up the chain to the GM and ask that they take the time to swap all 4 tire/rim assemblies with same vehicle on lot. Noise was gone immediately. They then can say it was the rims and replaced all 4 and came to the conclusion that it was strapped down too hard on the transport. Stay strong and forceful the longer you drive and days go by the easier it is for them to put it on you for a pothole or something.
 

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I had the same issue with a car I bought new a few years back. It got more pronounced after 500, 1000 and 1,500 Miles. After bitching up a storm dealer put 2 new front tires on. Magically noise was gone but started again after 500 Miles. I was furious. I demanded that the measure the rims for roundness and uniform width. Told me not possible on a new car unless I hit something. F that. I had to run this way up the chain to the GM and ask that they take the time to swap all 4 tire/rim assemblies with same vehicle on lot. Noise was gone immediately. They then can say it was the rims and replaced all 4 and came to the conclusion that it was strapped down too hard on the transport. Stay strong and forceful the longer you drive and days go by the easier it is for them to put it on you for a pothole or something.
Add to that over-inflation. My car came off the transport with 48 PSI in the tires. Some dealers with other makes have seen as much as 60 PSI. Not good for the tires. Those elevated pressures can cause stretching and belt shift. Complaints about rim distortion can be easily seen on a balance machine but, some service departments are adamant that it can't happen with a brand new wheel rim. Good prep people often catch these issues on the test drive.

-John
 

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My father had the same issue with a brand new 2006 300c. It had Goodyears that were made in Japan but not up to spec. He replaced them with a set of Michelins and the problem was instantly gone.
 
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