Muffler shakes!!

[amer]

I have a 2k2 max, japenese market model...

last night my friend noticed that the muffler shakes when I start the engine.
I asked him to start the engine and I'll look at the muffler, it shakes when " starting " the engine then it calms down, just like if something is blocking it and then let's the exauhst come out, there's also a very very low strange sound coming out of it, just like if somthing is poping inside a metal container.

auh yes, there's also something, today my little brother told me that when I started the engine a black smoke came out!!

Tomorrow I'm taking it to the dealer to fix it, cause it's under warranty, but I wanna know what's wrong with it.

thank you guys!