As a general rule, a car's performance will always decrease as it gets older in age. You cannot expect to buy a car brand new and still have the same performance 8 or 10 years down the road. However, you can get back the best performance possible out of your car with routine tuneups. Oil should be changed about every 3000 miles if you drive in the city most of the time, 5000-6000 miles if its mostly highways or every 6 month (whichever comes first). Routine tuneups are very important. I drive a 22 year old car and my mileage only dropped 4-5 miles when it was new because I've kept it up.
If its been over a year and you haven't tuned up your car yet then I advise you should do so as soon as possible. Change your spark plugs, spark plug wires, air filter, oil filter, fuel filter, accessory belt(s), distributor cap & rotor, timing chain/belt. All of these can affect your gas mileage if even one of these things are worn out overtime. As for the "tapping" noise, I cannot know exactly what may be causing it. I would assume from the best of my knowledge that it only happens when the car isn't fully warm yet as you stated it could only be heard it when you start the car. Maybe the culprit is that your timing is off and it creates a "pinging" noise.. or possibly that your belt or timing belt/chain is worn.
Even then, I would highly recommend you to go to an auto shop that does diagnostics. It doesn't have to be a Mercedes dealer, quite frankly, I think dealer cost are overpriced. A local diagnostic center should have cheaper price. And if you haven't tuned up your car for over a year, you should do it as soon as possible. Maybe after a tuneup, the "tap" would go away but either way, you will definitely see a performance gain in your gas mileage and horsepower.
