I noticed a slight bump in the driver seat in my 2009 Sonata when braking. It's not severe, but noticable. I'm not sure if it's just a bolt that's not tightened perhaps? I see others have similar problems.
My car has an automatic transmission.
There are a few different possible causes for the seat to have some play in it. There could be a loose mounting bolt, a seat anchor that has separated from the floor, or the cause that I’m seeing quite a bit in my research, is a seat track that has worn/broken and is allowing the seat track to move up and down slightly. I would recommend that you first check all of the mounting hardware to be sure that it is tight, and that the anchors have not pulled away from the floor. If everything is tight and secure, you can have someone grab the seat by the backrest and try pushing and pulling on it, with medium force, to see if you can identify exactly where the play is coming from. If you are unable to identify the cause, or if you would like to have someone diagnose the problem for you, I would suggest consulting a certified mechanic, like those available at YourMechanic.
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