If the buzzer is coming on with things it shouldn't be activated by, it usually indicates you have a poor ground somewhere, probably behind the dash or in the instrument cluster. The 124/late 126 buzzer is transistorized and highly sensitive to crappy grounds feeding back through it. My SDL had this issue, shifting into reverse, hitting the brake pedal, opening ANY door with the headlights on, you'd get this sick squealing noise from it that just wouldn't stop. Sanding and cleaning the ground points behind the cluster and the center console fixed the issue in my car.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)