There should be some water down in the trap which acts as a barrier to escaping gas. Sometimes when it has been dry for a long time the water in the trap will evaporate. Try pouring a few gallons of water down the hole. This should fix the problem until next time it drys out.

I too had a similar problem. My plumber installed an adjustable ball. When i get water in my basement the ball compresses to let water out, and when it is dry-the ball keeps the smells out. Works like a valve I guess. Either way the smell is gone.