Rattle coming from plate under car

Exhaust system loose

I suggest you have your car checked as soon as possible - while a rattling exhaust isn't desperately urgent it could be the cat breaking up which might later restrict the flow of gases. Most importantly excessively loose timing belts or chains tend to make most noise when the engine is idling. If the noise is being caused by this delay could be very expensive.

Are any of the speakers loose?

Chances are you have a bad sway bar end link, and yes they do tend to rattle

Bearing Plate on top of Strut,maybe

CV Joints snap & pop,don't rattle

Very hard to know without actually hearing the vehicle but,,,With what you describe,the cat/converter 'may' be making the sound.With the car off and engine cool,get under and give the cat/con a really give shove or hit.The insides of them do corrode and the internals make a very loud rattle.If this isn't the problem it will let you see if there is a loose exhaust mount that may also make a rattle.(The cat/con sound is like there is a very loose metal plate hitting something)Its actually the internals of the cat. that have broken loose and rattling inside it.Hope this helps

sounds like the baffles have come adrift in your exhaust sylenser

you could have a loose heat sheald or a broken catilitic converter you should take it to a shop and have it looked at tobe safe

More often than not it is the license plate. Any reputable audio store will have a silencing kit.

the "rattle" will be detonation caused by spontaneous combustion from incorrect air/fuel ratio and incorrect ignition timing
it is most prominent under acceleration or under load ( up hills , high speeds etc) and will melt pistons if left unattended
run the fault codes to find the faulty sensors , but I suspect that the heated oxygen sensors(HO2S) in the exhaust manifolds before the cat converters will be at fault as they report the gas composition back to the ECM so that the mixture can be adjusted correctly ( indicated by the poor economy)
there are knock sensors on the block which are indicated as faulty as they should be reporting this condition to the ECM to adjust the settings
finally it could come down to low fuel pressure or faulty injectors
what ever it is you need to get a proper fault code reading and have the problem fixed asap as it will cause engine damage if not fixed