everytime i start the engine on my 2000 tahoe the engine sounds like a tractor but once it warms up it stops... does anyone know what causes that??

 

Assuming a 5.3L this vintage engines used pistons that are not entirely round until after they got fully warmed to operating temperature thus causing some noticable piston slap in the cyliders when cold. If I recall it had something to to with being "low friction pistons" as GM stated and were probably used to help meet the federal CAFE standards for fuel economy at the time. GM took a lot of heat about this issue back then but kept telling customers that "it's normal and doesn't hurt anything, just don't race the engine cold". Bottom line is with your Tahoe being as old as it is, if it runs good and doesn't use oil then there's probably nothing to be concerned about. If it was going to cause problems it would have happened a long time ago.