I noticed this while my friend was playing it.

He went out today to go get it and my friends and I all watched him play the first level.  Almost everyone commented on how it was chugging at certain points.

I haven't heard any of my friends complain about this.  I would test but I'm not a Halo fanatic.

are you asking for help or just want to let us know?

Yup I got this at the start but seems to have gone now.

Hmm.  Different models of 360's, quality of disc drives, or disc vs. HDD, I guess.  Sorry for your experience.  I hope it doesn't happen when I play through it.

Happens on the first level when you first encouter the covenant. That's about it. Performs much better later on with lots more stuff on screen.

It had me worried at the beginning.

I just played a section with a lot of shit hitting the fan, and everything was smooth.

The only hiccups I encountered were during some of the cutscenes.  I had no noticeable hiccups while playing though.

it happen ridiculously in the level evacuation 

it all depends on the level . the first second and 6th levels all perform horribly while the 3rd 4th 5th are perfect