1988 Chevy Silverado Leaking Fluid

I recently bought this truck from a friend. Owned about a month. I went out to warm it up. The vehicle ran about ten minutes. I went back outside to get in and leave. Noticed a slight sound under the hood. When I looked under the hood, transmission fluid was literally running out of the fill/check spout. I have been around motors all my life and have never heard of this happening. When I cut the vehicle off, of course it stopped. I had checked the fluid about a week earlier and it was at the perfect level with the transmission hot and the auto running. Now about a week and a half later, the same thing has happened again. I asked some more experience people, one an actual mechanic, no luck. Any suggestions??