I have a 2002 CLK 430, there is a noise (tone) coming from the truck, anyone know what this may be?

Have to be more specific, could be anything...

Its sounds like a tone that you may get when a gas cap wasn't tighten enough. I tried to re-seat the gas cap but it didn't stop the noise. the cap doesn't look worn.

Well I am taking a shot here, a wheel bearing going bad will make a tone, so to speak, never heard of a gas cap making noise, but if it is in the back of the car, get the rear end grease checked.

a little more info, this noise isn't present all of the time. most of the time it starts after acceleration and only goes away after I turn off the engine. there is no decline in performance or gas mileage.

OK, then the noise is coming from the engine? If so Idler pulley or belt tensioner pulley do go bad, there part of the system that the fan belt, (serpentine belt) rides on. Also alternator bearings can make noise. Best to check by taking the belt off and spinning the pulleys, inspecting the pulleys for play. Need to determine where the noise is coming from...

So what you are saying is the "tone" is not just when the car is moving, but also when the car is stopped? Than it may be a bad radio speaker? or a hole in a muffler or exhaust pipe? or a muffler clamp lose depending on what you mean by "tone". Is it a whistling, hissing, clanking, bubbling, murmuring, thumping, or bumping or static tone? No bearing will continue to make noise if there is no movement on the car. Do you have air shocks and it may be the air compressor? Have the shocks checked for leaks if that is the type of suspension system you have. It just may be the compressor leveling your parked car. My Buick did this all the time. Not only did it level the vehicle when parked, but it kept the tank pumped up if it went low after parking it.