Loud reverberating whining noise coming from dashboard

Roughly 5-10 minutes after start (seemingly on colder days) my 1978 300D emits a loud reverberating whine that seems to come from the driver's side of the dash. While intermittent, it is quite unsettling. The best description I can give for the sound is that it is similar to the sound one might hear from a poorly lubricated fan (where the grease in the motor is caked with grime). The sound happens without either climate control or defrost on. Is it A/C related or something more serious? I know the vacuum system is in that general vicinity, but what else might be causing the problem? You can either respond to this thread or write me: jimbackus@hotmail.com Thanks!

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Questions and Answers

Chassis Model Number

W124

A little more info needed

Does the noise happen with the key on and engine not running or only when the engine is running. Does the sound change at all when you accelerate? Does it stop immediately when you turn the engine off. Any other symptoms?

Info supplied!

Thanks for quick reply! The noise happens when the engine is running (and roughly 10 minutes after ignition) but unfortunately every time it's happened I've been in heavy moving traffic and unable to pull over and turn off the engine (such is New York City driving). By the time I am able to do so, the sound is gone. On average, the noise lasts 1-2 minutes and seems to increase in volume before abruptly stopping. Sometimes it starts again, only to stop shortly thereafter. The noise maintains a consistent pitch whether I'm on the gas or not. On one occasion it continued to climb in both pitch and volume then suddenly stopped altogether. Pressing/releasing the accelerator did not affect the noise. While hearing the noise, if I place my hand on the driver's side dashboard (near/over the speaker), I can feel a vibration that is in-sync with the noise. A passenger noted that they could feel a similar vibration in the passenger seat as well. Today it started immediately after I hit a sizable pothole. Whether a jarring causes the sound is not known since the sound is intermittent (and potholes abound in NYC) and seemingly confined to very cold days (sub-freezing), although again I can't provide evidence of the latter statement. Turning off the radio makes no difference (I had thought it might be a bad ground causing the radiator fan to 'talk' through the dashboard speaker, but that is belt-driven, no?) nor turning on/off the defrost. The first time the noise occurred was on a very cold night nearly 1 month ago and it hadn't happened until this past weekend and most recently today. Both recent days were similarly colder than normal (well below freezing) but again, I can't confirm that this is indeed a necessary condition. That is all the information I have on this mysterious sound. Any thoughts or help are greatly appreciated. -Jayson

AC Unit

I think your ac unit is squeeling. The unit can be quite noisy and travel up to seem like its comming from the dash. Especially on turns. Make sure your belts are tight. And in the winter, run only defrost and the ec setting. You can disconnect the belt it you like- I did. I think the AC start to 'freeze' up over time and the cold can really affect them. Just my thoughts