After doing some research over at mbworld and listening to the engine with a scope, I have discovered the following:

Yes, on the driver's side fender there is a small black component known as a purge valve. Mine is on intermittently and makes a moderately loud snapping sound that does change with rpm. It is noticeable with the hood up, and once you hear it closely you'll also hear it with the hood closed. You can feel it pulse too. That may be part of what I hear, but the main noises are coming from underneath the covers stamped with SRT-6. I'm assuming the fuel injectors reside there. If so, they are noisy as hell! Some of the MB guys have used fuel injection cleaner to quiet them down a bit and some have paid 170 bucks for a carbon flush to remove carbon deposits from the valves as well as the injector nozzles. Any thoughts?