Cat came apart...check that while your greasin the bearing...

 

OH GOD!!! I CANT STOP LAUGHING!!! OH MAN!!!!

Dude, just get under your truck( put it on jack stands if you need to) and just check the exhaust pipes from as far as you can reach up to your manifold on both sides and then all the way back to the muffler. It sounds like one of the heat sheilds on the cats or muffler came loose just use some wire or a coat hanger and tie it back on.

 

Look for loose heat shields.
Whack on the converters. I once had the innerds of a converter break loose and they made lots of noise.

 

You could need new muffler bearings they could of blew apart and are rolling around in there, I'd take it to your mechanic...

 

Just grease the muffler bearings at the grease fitting, yea hope it's not ur cat either, might need to grease ur cat bearings too. Just grease those the same way as the muffler bearings.

 

Is it special grease?

 

If it only does it right after you start it then goes away i dont think it would be anything loose other wise you would hear it all the time. I bet the tailpipe is just just cackling

 

Yes, easy to get. Just go to your local auto parts store and ask for it. They usually keep it behind the counter.

 

That sh#t just ain't right. Lol

 

The flux capacitor on my truck blew out a couple weeks ago, right after I changed my muffler bearings... It's a common problem, you should change that as well. The part store should have both