weird rattle-grind noise when braking.

[krolos]

I have a weird rattle-grind noise sometimes when braking, mostly at slow
speed and coming to a stop, it seems to be coming from lower engine
compartment but hard to say maybe more to the passenger side,
it is not really loud, and it is not a gear grinding kind of a noise.
This only happens periodically and I can go days without hearing anything,
or I do some test Runs and it wont happen,but it is definitely not normal
it is still under warranty, but kind of tricky taking it to the dealership when
it only happens periodically. I have now 33,000 on my 2008 Auto sedan.
anyone have idea what it might be.

Thanks.