when im between 10 and 30 mph i hear a rattling noise, its really quiet i have to have the blower and radio off, and i can hear it, and can really hear it with window down, this is new, my trucks got 154,200 miles, its getting old but the engine sounds and feels great despite the rattle i dont believe its the internal engine, plus its to clear of a rattle to be from inside. i baby my truck so this is a problem and i deliever pizzas so it needs fixed before i get screwed on a delivery. my belt started squeaking it squeaks from the fulley that spins the fan, i was thinking maybe its my belt, i think its bad pulley or a loose belt, how tight should the belt feel? if i had to guess id say its the pulley that doesent serve any generating purpose, idk it looked like it was shaking when in idle. i really have no clue though when it comes down to it? its a 1997 chevrolet blazer 4x4 lt, 4.3l vortec high output.

 

If you haven't figured it out, check your catalytic converter. Idle the engine and crawl underneath. I bet your converter is the culprit