We have found that a few things rattle on our stove. We do not have individual grates on each burner, we have one large grate that covers all 3 burners. We do have a cover on ours. The cover rattles and so does the entire stove top. The stove top just sits on the counter, it's not fastened down.
snowdance's suggestion works well with individual grates. We just put a towel between the grate and the cover and that helps a lot. Not much we can do about the entire stove top from rattling or bouncing over bumps and pot holes.
-Michael