Irregular Idle, Knocking Sound: 99 SLK230, Need Advice

Thanks for taking the time to read...
I just purchased a '99 SLK230 with 34,900 miles with a superchaged 4-cylinder engine. The car appears prestine, but after driving it for
a few weeks I am starting to become concerned about the engine. When I inquired on the vehicle service history, the one owner before me never had any maintenance other than oil changes performed. So I paid the Mercedes service center to perform a Flex B service (inpect, top off, etc..) and asked them to pay close attention to the sound the engine is making. They changed the oil, and wiper blades and $400 dollars later I got my car back with the same irregular idle and knocking sound in the engine. They said they didn't find anything out of the ordinary - not what I was expecting to here.
I'm not an expert by any means, but I've owned a lot of different cars. All of my well maintained car's idle were smooth and steady with no knocking sounds from the engine. I'm also convienced the car should be faster than it is - 185HP in a 3,000lbs body should be zippy, but my 2003 4-cylinder Mitubish Outlander SUV (which I traded because I hated) appeared quieter and nearly as fast as the Mercedes I just bought.
I want the car to idle smoothly, get the knocking knocked-out, and perform like a sports car. I'm not convinced the Mercedes dealership service center gave me a fair evaluation of my concers. Can anyone provide advice on how I can get these things taken care of without spending $180 per hour at the Mercedes service center that doesn't think there is anything wrong to begin with? Or should I just accept that a now over 4-years old car with now 35,100 miles should idle irregularly, make knocking sounds, and perform under par?
Thanks, - Matt
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My 230 is 5 years old (just bought it) and it doesn't knock and is fast. You have to drive it fast though, I.e put your foot to the floor when you want power.
On 4/1/04 5:06 pm, in article snipped-for-privacy@posting.google.com, "Matthew Pomar"

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You're right that a 185 HP engine should move a 3,000 car smartly, and smoothly.
Someone has to check that the engine's specifications are set correctly. That the engine's ignition timing is correct, that the compressor's boost is to specification and that the idle mechanism is clean and properly adjusted. None of these checks require opening the engine, they require a mechanic who's knowledge of this engine is good. This isn't complicated so don't be "blown off" by some service writer salesman who wouldn't know a piston ring from a crankshaft.
You asked for a lubrication service and that's what was delivered, no one even looked at your real complaint. Life is a learning experience, next time, don't sign the estimate unless you fully UNDERSTAND what, exactly, will be done. And NEVER just say "fix it" without first KNOWING the cost. I've learned from experience.
You want the car to perform as advertised, it will after these checks.
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You might also make sure that you or the previous owner was/are using premium fuel..Using regular will cause knocking.

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