Problem when accelerating

ok here's the deal, i have a 5spd 1990 5.0 with the mods in my sig. Everything drivetrain specific has been rebuilt with less than 10,000 miles on the parts. Whenever i am at a complete stop and start to go almost immediately after i put my foot on the gas pedal i hear this loud "clonk" sound come from underneath the car. Now it didn't start until i got my gears installed. but my dad who is a mechanic for Goodyear took the car to work checked them out and couldn't find anything. Yet it still makes this sound with no progress of getting worse or better. Also it only makes this sound when i take off at a slow rate, when get ready to drag race and launch hard, absolutely silent. I haven't felt anything unusual while driving like jerking or anything like that. does anyone have ANY idea what the hell is going on? i guess it's more of an annoyance than a real problem but still. :banghead:

Is the sound coming from your Diff???
I had A ford dealer (incompetent) install my 3:73 in my 98gt and a few days later it started clankin" like you described...Found out the pinion wasn't seeted all the way and was barely loose.....had to have it pounded in again at another shop.....??? mabey this is it??

Did your place the driveshaft after the gear install with a nice alum one? Also check for bad bushings on the control arms and such areas.

Is the sound coming from your Diff???
I had A ford dealer (incompetent) install my 3:73 in my 98gt and a few days later it started clankin" like you described...Found out the pinion wasn't seeted all the way and was barely loose.....had to have it pounded in again at another shop.....??? mabey this is it??

that's what i was thinking the differential, but i'm not sure. thanks for ur input i'm gonna probably look back into that.