In my 87 300sdl ther is a buzzing sound in the front left speaker when the radio is on and I turn on the head lights. When I move to the rear speakers the sound goes away. I have had problems with the battery going down, when left for a week or so. Is the short in the switch or the radio? Thanks for any help.

Check the radio suppression spring (clip) in the left front steering spindle.

Thank you, Iam not sure what the spindel is and how to get to it.

Is the buzzing heard while driving, while idling, or with the engine off?

When you say "When I move to the rear speakers the sound goes away," do you mean that you can't hear the buzzing in the rear speaker or that the buzzing in the front speaker goes away?

The sound comes only from the left speaker. It's there with or with the engine running.

your speaker may just be blown.

Sounds perfect without the lights on.

Mine does the same thing, but I never bothered to try to narrow it down to which speaker.
I was at the library one day lurking in Alldata (pretty useful sometimes) and looked up TSBs for the SDLs. there was one from early on, like '86 or '87 about buzzing speakers when the lights were on. If I remember, the fix was to insulate one of the speaker wires in the dashboard - I assume it was getting interference from a nearby headlight wire. Near as I could tell there was some mercedes approved speaker wire jacket thing that would supress the interference and humming.

Thanks,. that makes sence. Can you get to this wire from under the dash or does the radio need to be pulled?

Mine does the same thing, but I never bothered to try to narrow it down to which speaker.
I was at the library one day lurking in Alldata (pretty useful sometimes) and looked up TSBs for the SDLs. there was one from early on, like '86 or '87 about buzzing speakers when the lights were on. If I remember, the fix was to insulate one of the speaker wires in the dashboard - I assume it was getting interference from a nearby headlight wire. Near as I could tell there was some mercedes approved speaker wire jacket thing that would supress the interference and humming.

There's no way a low impedance (4-8ohm) speaker is going to reproduce any noise induced from any wire(s) in proximity.

The noise has to be introduced in to the system 'pre-amp". Ground loop, broken cable shield...

Does this system have an external amp?

Yes, the black box in the trunk

And, It is only coming from the one speaker???
I would look at the cable from the radio to the black box. The buzz is most likely alternator noise which will be greater when it is loaded. (headlights on)
This noise is riding on all the 12 volt lines but is very, very minute. A very small fraction of a volt.
Now, The average sensitivity of a 4 ohm car speaker is around 88 decibels with 1 watt going into it. 2 volts would equal 1 watt. Unless your alternator has some serious issues the noise should be millivolts. The only way to hear the noise is for it to be introduced BEFORE the amp. The amp does it job and amplifies the noise along with the other noises (known as the hidious music that gets airplay today) and is heard in the speaker. How this is only occuring in one channel of the system is a mystery.
Did it just start?

It's been like this for some time now. I replaced the alternator about 6 mo. ago, and it's still the same.

I have exactly the same problem on my '87 300SDL. Not sure which speakers, but I hear hum while sitting in driver's seat when headlights on, battery drains in about a week. Bought car a year ago and mine was this way when I got it. Have not measured the drain or pulled fuses yet. Hope somebody can figure it out.
Can someone explain what "Check the radio suppression spring (clip) in the left front steering spindle." means?

I'll take a stab at the radio suppression spring comment.

MB, as well as a few other manufacturers, equipped cars with a little spring/clip thing under the wheel bearing cap, in the center of your wheel.
This little clip is stuck in the middle of the spindle once your spindle nut is installed, but before the grease cap is installed. It provides a ground path from the spindle, without having to try to conduct through the bearings.

Weather it actually achieves much, who knows. Most makers seem to have gone away from this practice, but that may be in the interest of cost cutting as much as anything.

Here's a pic of one:
http://www.mercedesshop.com/shopforum/vbpgimage.php?do=full&p=1887

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?

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.:book: 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?

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.

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.