If the vibration is felt in the seat or appears to be the entire vehicle, then the rear is out-balance or out-of-round.
In your case, the vibration is felt only during braking, so the most likely cause is due to thickness variation in the front disc brake rotors, commonly called “warped rotors.” Rotors can become warped over time due to rust forming between the hub and the rotors or by overheating the rotors, such as during braking when descending long grades instead of using the gear selector and using low gear.
Another possible cause can occur when the wheels are tightened using an air impact wrench instead of a torque wrench. The rotor can become slightly deformed by the unequal torque on the lug nuts. All wheels should have the retaining nuts tightened using a torque wrench.
The situation will likely continue to worsen but is not likely to be a safety issue. There are a couple of options including:
- Have the rotors machined. The technician should thoroughly clean any rust between the rotor and the wheel hub and tighten the lug nuts using a torque wrench to factory specifications.
- The rotors could be replaced instead of being machined, often called “turning the rotors.”
It might be wise to have the disc brake pads replaced at the same time because the technician has to remove the disc brake caliper to get access to the rotors anyway.
Depending on the number of miles on the vehicle and how long it will be kept will help determine what is done. I would recommend taking it a shop or dealer and asking for their opinion and recommendations.
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