I have found a couple threads about this issue, but all of them either do not really match my symptoms nor come to a helpful conclusion. This is about my 1985 300TD Turbo.

I have been having a loud distinct whistling at certain rpm. It doesn't coincide with the engine speed. It is most distinct at third and fourth gear. As it gets louder, it seems to take some power away and slows the car a little. Or, if I let off the gas a little, it will also become louder. It will get louder and the car becomes slower (maybe 5 mph slower, typically) until I push down on the accelerator and the whistle silences and it feels like the turbo really spools up and the rpm increases and off I go with a bit of power. If I stay cruising a ~65 mph, then it never happens. But, at around 45-50 mph, it gets very annoying.

The previous owner told me it was a vacuum leak, other similar threads seemed to propose it was an intake/exhaust leak. Something else I just read talked about bypassing the EGR, maybe there could be a leak there?

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