Hi guys, thanks for the replies as this is my first time trying something like this.
I don't think it's the alternator as the squeal/whistle is only for a second and is always the same pitch. My brief experience with previous alternator failures is that the pitch goes up and down with engine rpm. This squeal stays the exact same pitch (or tone). It actually is close to the pitch of a brake squeal but of course isn't the brakes. I've also jammed the brakes when driving the car when the noise starts but it doesn't affect it. I also can't get it to squeal at low rpms, only at 3000 rpm and letting off on the throttle. I have heard the noise repeating 3 or 4 times when coming up a steep hill and thought for a while it was the radiator cap so changed it with the same correct Mercedes temp spec but no luck.
I have been able to make the noise when revving the engine in the driveway and letting it drop in revs but of course a diesel doesn't decelerate that quickly when there isn't a pull from the car so it's hard to get it to do it every time. My 11 year old points to the top part of the engine and when trying it myself I can tell that the noise is also near the EGR. I changed the 1/2" hose leading under the EGR through the U shape in the air duct to the air cleaner but it didn't change anything and the original hose was in pretty good shape considering the age.
The EGR has a black dust near the vent hole but I'm not sure how to test it to make sure it's sealing correctly. I'm told that the 1985 300SD followed the California exhaust emissions so the EGR etc is set up different that the 1984 standard US (non California) model. Not sure if this is true but web sites seem to indicate this.
It seems that pressurized air is not being needed when you lift off the throttle and it is being expelled like a steam whistle. Does this still point to the EGR or something else? If you still think alternator, I'll pull the belts and try to repeat but it typically won't sound unless the car is up to full temperature.