Sounds like a relay clacking at a high uneven rate of approx 10 x per second, but only from a cold start then it stops after a few minutes. Is this a common problem? I.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

A lady I knew with a 82 240D had a "buzzing" coming from behind the radio, which she ignored and went in the house for the evening. In the morning when she came out to go to work, it looked like someone had "tinted" her windows during the night. Oh yah, totolly toasted inside. I would disconnect the battery till you figure it out.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

My 85 does the same thing, sort of a buzzing almost, lasts about 5 seconds then in gone. I suspect a vacuum control becoming active but I dont seem to have leaks anymore so why a control would change after a bit.... Only on cold starts, usually if it is below 50f, and come on around 60-90 seconds after starting. It did not do it this morning Weird stuff...

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1985 300D
The rest:
1957 MGA (comatose)
1965 Falcon (sleeping)
1966 E-100 (rust test in progress)
1976 Ford 3400 D Tractor (workhorse)
1978 Mercury Zephyer (5L playtoy)
1995 Isuzu NPR D (fetcher)
1998 Subaru Legacy (Spare)
2000 Toyota Sienna (School bus)
2008 Toyota Prius (Commuter)