Some other observations:

The hum is the same loudness regardless of radio volume setting. Same in either radio or cassette mode.

Hum is present without turning key on (i.e. engine off) or car moving if radio on and lights on.

Hum produced when parking lights or headlights on, but not when rotating light switch left for left only or right only parking lights. (not sure what those are really called)

When radio volume turned down, hum seems really loud, but when volume up not very noticeable with radio audio. Especially with highway noise.

Could the circuit that detects burned out bulbs be responsible? Anyone know how it works?

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Joe Myers

'83 300D (light blue)
'82 300TD (off white - parked)
'87 300SDL (silver)

'75 300D (black - sold)
'76 300D (beige - totaled by pizza delivery car)
'82 300D (metalic blue) sold

Good. We can rule out the alternator.

Only the lights affect it? What if you turn the AC/heat blower on with the lights off?
That will determine if its a voltage drop or something coming from the headlight circuit. I would expect the voltage to drop with the blower 'on' just as it would with the headlights. Do this with the engine not running.

You may be on to something, IMHO start at the idiot buzzer that tells you that your lights are on, this circuit doesn't involve the right or left parking light switch.

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85 300CD The FryrBird MY daily driver

The Hazard blinkers and sun roof do not cause the hum. The Hazard blinkers are much brighter than the parking lights, so current draw should be higher. I did not hear any hum with blower on low. The high blower was so noisy I don't think I would have been able to hear it. While glow plug light is on I hear a much lower level hum. That hum stops when the glow plug light goes out. But never heard the louder hum from the glow plugs. I "assume" the glow plugs are still on for at least a few seconds after the light goes out (without starting engine)? So does not appear to be just high current draw. Maybe this will help somebody figure it out.

__________________
Joe Myers

'83 300D (light blue)
'82 300TD (off white - parked)
'87 300SDL (silver)

'75 300D (black - sold)
'76 300D (beige - totaled by pizza delivery car)
'82 300D (metalic blue) sold

Based on this info I'd say it has to be something on the light circuit. Weird.
If you were close I have an O-scope and we could possible track it down.

my car has the same hum to it, and there's no radio in the car. seems like it the noise it coming from the control in the center console. plus, mine gets worse after a storm -water get in to it because bad seal in the window. i was going to bypass the old system when i get a radio.