my 300sd did that. My mechanic said it was some sort of amplifier gone haywire. he reccommended i not replace it because it was too expensive, Of course with him if it wasnt from the dealer he would not buy it or install it on my car.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual