1993 Mercury Topaz buzzes and a puffs of smoke

1993 Mercury Topaz mileage: 134,000. This problem has happened four times over the last two weeks. When the car is idling (like at a stop sign), something under the hood buzzes and a puff of smoke comes out. It began as just a short buzz that repeated itself once, but the last two times the buzz has increased in length and the amount of smoke given off increases. The smoke is gray, not black. I had my alternator checked, and it is supposedly okay.