When I gun it in my T/A, when it hits third gear (I think) there is this THUMP mixed with a GRIND, what the hell is that?

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It's all fun and games 'till someone looses an eye....then it's fun you can't see!!
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1987 Pontiac Trans Am
-305 cid motor
-700-R4 Transmission
-K&N Air Filter
-Flowmaster 40 series
Muffler
-Yokohama Tires
-GTA Rims
-3" Exhaust Tips
-180 degree Thermostat
-2.73 Posi Rear End
-JVC Head Unit w/ Kicker subs!
-And a COOL LOOKING Flowmaster sticker on the dash!
WOO-HOO!!
*Long live the F-Body!*

 

That sounds like your tranny mount is completely worn down. Get a new mount, preferrably a poly graphite one from Summit Racing.

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- Mike Harvey -
1986 Firebird S/E