My 600 makes a grinding noise beneath my feet whenever I press the gas hard?

Sounds like it could be the cat or some other part of the exhaust system making contact with some other part when the engine "torques" under acceleration. Put the car on a lift and try jiggling the exhaust system to replicate the noise. (It could be that the noise is not coming from beneath your feet, but is being "telegraphed" so that it seems like it.)