I drove home last Friday and when I turned the engine off, there was a loud buzzing sound coming from under the hood. I couldn't figure it out but after a few minutes smoke started coming from the engine. I took off the battery terminal and the buzzing turned off. Before I did this, I tried turning the engine back on but the engine wouldn't turn over. I also observed the temperature gauge and it was around 80. I put back the terminal today and the buzzing sound came on again. Any guesses what this is.What should I do HELP!!!

THANK YOU FOR YOUR HELP.

1984 300d Turbo- 370K