Need more info, what model or engine do you have? What is the nature of the noise? Exhaust or mechanical clattering? Try to be more descriptive and hopefully we can help. If you don't think the car is in imminent danger of grenading you may consider the following.

You might start with running a couple of cans of seafoam through it to start the process of getting it up to speed. That will help condition the fuel system. Its also relatively cheap to swap out the plugs/rotor/cap and ignition wires that is a typical service that is often overlooked and can have a dramatic impact on driveability.

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1981 300TD
2005 C55
2004 E500
1999 ML430
1992 400E
1998 C43
2001 SL500
1988 300SEL
1986 190E 16V
1991 190E 2.6