Just picked up my 95 300d from mercedes shop for glo plug replacement, new serpentine belt and a new disc. When taking the car above 40 mph, there is a loud siren like sound coming from the driver's side of the dashboard. No warning lights come on inside the car. When slowing down, it diminishes. Driving around town, this does not happen, only on highway. Could the mechanics have loosened one of the wires while they were in there? and, more importantly, is it dangerous to drive it?

It could possibly be a binding speedo cable. Does the speedo needle bounce around more at low speeds than it did? Also, does the the sound vary at different speeds? Try finding the cable and give it a gentle wiggle to see if that helps. You may need to lube the inner cable if that doesn't help. There's lots of info on that here in other threads.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

I have an earlier car which made a siren like sound at speed - turned out to be air vent in dash.

Did you check your rearview mirror for flashing red lights? :p

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Dean Albrecht
"Lead, follow, or better yet, get out of the way!"E500 owners motto