Turn Light on....Whining sound in engine

I have a 1990 300E $-matic. Recently experienced a problem where I turn the headlights on and a high pitched whine comes from the engine. Turn lights off,
no whine. Any ideas?
Thanks, JD
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Do you hear it only when you turn the lights on or will also happen if you turn on another high current item like the power seats?
If it happens when other high current items are turned on, it could be your alternator or belt.
Scott D

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Yah! Dat is der little mice in der engine kompartment vhat ist der to keep varm in der cold veather. All dat you need to do is put some little mice traps in and get them. They are the little black night mice vhat dont like der light.
mcbrue advicegivingly under the bridge in the trailer down by the river
96 S420
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Thanks for the reply guys. I don't know what happens when the engine is not running. I'll try it tomorrow. I was going to guess the alternator, but I would think that is charging all the time regardless of whether the lights were turned on or not. I would think it is trying to get the juice back in the battery from starting the car. I'll also check the belt to see if it is tight enough. We just replaced the serpentine belt less than 6 months ago. Also, I feel so important now that McBrue has weighed in on my small problem. With 144k on a 4 matic, I don't have much room to complain. I just hope I can nurse the 4 matic transmission transfer case for another 50k or so. I appreciate your input guys.
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is that with or without the engine running...if with...it's probably the alternator...(bearings)
your headlights use a lot of power...which used ones sometimes have a hard time supplying. what's the mileage? a change of drive belt may help momentarily
alx

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