Over the past two days I've taken 4 long (2hr) highway trips (~130km/h) in my 86 300E. After two of these trips, I noticed a distinct and fairly loud buzz from underneath the car when I slowed to idle and rolled down the window. Turning off the engine and then turning the key to the on position (but not starting the car) again produced this buzz. I usually hear some something powering up when doing this - always assumed it was the fuel pump but I'm not sure - and the buzz seems to coincide. Any advice?