If you have an MTX then the rattling is probably just the gears. Changing your MTX fluid might reduce the noice, but if you haven't ever changed it, you should definetly do that.
Check your auto parts stores for a "3 step intake cleaner kit" (I used the kit from Berrymans). It contains throtlebody spray cleaner, intake manifold + chambers cleaner (via vacuum line), and fuel additive to clean the injectors and further clean the chambers. This is almost as good as removing your upper/lower intakes to clean them. I removed mine, but that didn't help clean out the chambers.. the kit made them all nice and shiny.
If you haven't replaced your plugs and wires you should also do that anyways.
I've seen people here recommend changing your oil and plugs AFTER doing the 3 step intake cleaner... which does kind of make sence.
If you haven't done any of these previously, you'd had better notice a difference after doing them. If you're still having the bad gas mileage problems... might want to test/replace all 4 o2 sensors (as the other guy recommended).
