it varies a lot but sometimes when i start my truck a loud thud, like something hit the undercarrige occurs, but then again it doesnt always do it either, any ideas? the truck still runs fine, it has done it for quite awhile too

 

Check the starter and the flywheel... starter could be loose, flywheel could be broken.

 

it has to be the starter, cause i put a new TC in not too long ago and the flywheel was fine, and it did it before i put the TC in