On my 2001 C320, ill get in my car in the morning and start up my car, it idles at 1200 for about 20-30 seconds then slowly decreases the RPM,, When it gets to 700-800 RPM, there is the low frequency vibration in the car that is in some wierd way audible almost like a faint low bass sound you would get from a sound system.....

However after 10 minutes of it idling it goes away,, Does anyone know what this is, or how i can correct this??

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2001 Mercedes-Benz C320 3.2
1991 Mercedes-Benz 300E 3.0