getting a rattling sound under my truck under load...

your cats dont actually have something lose inside that you would be able to hear when you hit it like you did. the reason the cats make these noices is because of the catalytic chemical changes going on. this is most likely due to a almost completely clogged cat (still allowing to pass emissions). if you trust in us to replace the cats, you're sound will go away.

 

I just replaced my driver side hub....basically when your going down the road its a really loud hum...and i noticed it when i would slow down from 30+mph it just sounded like something was a loud hum till i came to a stop, then it started grinded one day and thats when i immediately took it to replace it. also if you shake your truck side to side and see your front tire going side to side with the truck...thats a big hint you need a hub. my front tire would go back and forth a full half inch which was scary