What causes explosion sounds but no explosion?

I built an IC2 crop harvester outside ( kind of has to be, but IC2 machines don’t mind rain ), and a GT transformer next to it. The first time it started raining, I heard lots of explosions and realized the transformer doesn’t like rain, so I threw a dirt block on top if it. That seemed to fix it but it still makes an explosion sound when it rains every now and again. Any idea why?

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