1979 Mercedes 300sd high-pitched sound when in second gear

1979 Mercedes 300SD Turbo Diesel mileage: 103,000. I've never owned a diesel car before, so I'm not sure what this could be. The other cold morning I started the car and I let it run for a minute then drove. As soon as it shifted into second gear, it started screaming like a high-pitched horn sound coming through what seemed like the vents into the front of the car. When I slow down, the noise lowers in tone until I'm in first gear when it stops altogether. I wasn't bold enough to try a higher speed. I'm afraid to take it to a mechanic without having some idea first, because I don't know any around where I live. (I just bought the car a few months ago.)