Slapping sound when shifting gears

2009 GT 5-speed. Hurst shifter, FRPP handling kit (shocks/springs/struts and stabilizer bars), and Steeda adjustable UCA. When I shift gears there is a pronounced slapping sound from the rear/center, around where the UCA bolts onto the frame. I checked the pinion angle after installing the UCA and it was within spec. Because I had never done it before, though, I took it to a trusted shop to double-check and to make sure I had torqued everything to spec. The sound is literally like someone slapping a butt--a big fat butt like mine. I suspect that something is actually hitting the underbody. First, has anyone else experienced this (the sound, not the butt slap)? Second, if something is hitting the frame how can I tell what it is when the car is moving? Finally, did I trust the shop more than I should have? IOW, should I take it somewhere else? Thanks in advance for your help.

First, this is in the wrong section.

Second, do you have a video of this sound?


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Get underneath the rear w/ creeper & check things out yourself...if this slapping's going on, there's got to be marks in the area(s) of origin.

07gts197 said:

First, this is in the wrong section.

Second, do you have a video of this sound?


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Sorry. Reposted in 2005+ forum. Old eyes.
No, don't have a video.

Iceman62 said:

Get underneath the rear w/ creeper & check things out yourself...if this slapping's going on, there's got to be marks in the area(s) of origin.

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Great idea. Will look tomorrow. Thank you!

wdrlaw said:

Sorry. Reposted in 2005+ forum. Old eyes.
No, don't have a video.

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don't repost threads... moved.

Have you ever replaced the bushing on the top of the diff, where the UCA bolts to?

07 Boss said:

Have you ever replaced the bushing on the top of the diff, where the UCA bolts to?

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Yes--Steeda UCA came with new bushings.

@Iceman62: Spent about 20 minutes tonight under the car with a bright LED looking for scrapes, dents, scratches. Nothing other than evidence of very light contact with new jounce stops. The car is about 1-1/2" lower at all four corners--I can no longer fit the jack under it with a hockey puck, unless I drive it onto a 2x10. No visible scratches on diff/axle, UCA, stab or panhard bars, driveshaft, exhaust, or chassis/unibody.

But I couldn't read what forum I originally posted in, so I might not have seen any marks.

@stkjock: Sorry for reposting, won't happen again.

This is a tech topic please keep on topic