I had this on my '85 as well. As stated, the whine/squeal goes away when you unplug the OVP. I've replaced all vacuum connectors & elbows on the two vacuum control valves mounted on the left front fender just above the glow plug relay (one elec. operated, one vac. operated), as well as on the connection to the EGR itself. This has minimized the whine but I still get some, at light throttle only, 2000-2500rpm or so.....exactly when you would expect the EGR to activate.

The whine on mine is pretty minimal now, but once the weather warms up I'm planning on tracing it down by unplugging the electric connector to the fender mounted elec. operated vacuum valve, unplugging and capping the vac line to the vac. valve right next to the EOVV I just mentioned, and lastly capping the line at the EGR connection. By doing this one by one it should be possible to isolate the culprit making the whine (which in my case at least clearly sounds like a vac leak).

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1985 300D 231K anthracite grey w/black int.