Sounds like you need a good local expert mechanic. Have him put the car up on a lift and carefully look for (and listen, with an automotive stethoscope), the source, prodding and poking and searching for something loose, with the car not running. Engine, suspension, underside covers, etc. My '07 has plastic under-engine covers, but I think yours has metal ones. One loose or missing screw could cause the problem. Loose bolts, steering components, anywhere a chain or hook could have been attached, too, as mentioned above. Then start and gun the engine and listen underneath to see if you or he can tell where it's coming from. Then do the same thing on the ground with the hood open, from the top. If it is not an external thing then it's an internal thing, like an alternator bearing, idler pulley, broken motor mount, bad water pump, power steering or linkage, suspension, etc. To test steering and suspension components requires knowing how to unload the tension from each part and then check for slack. But, if it's happening when the engine is gunned and the car is sitting still it is not likely steering or suspension related. My top suspects would be those bottom covers, motor/transmission mounts, or tie-down damage, which could, in an extreme case, even include an exhaust leak or damaged heat shield on the catalytic converter, exhaust manifold, or even a loose or broken off sensor creating an exhaust leak, or something on the transmission. A loose spark plug - but the engine would run rough then. Obviously it is something, so some finely tuned detective work should find it. Good luck, and let us know how it works out.
