Strange, LOUD noise after engine is turned off

[gmc396]

I have a '68 GMC 396 that I've owned for 7 years. It's a summer ride for me as I live in WI. I got it out last week and drive to get my mail daily. It's about a 5 mile RT.

When I pull into my garage, I typically turn off the ignition and let the truck roll into place. I do this to keep the exhaust smell out of the garage (the exhaust pipes exit just behind the rear wheels). Yesterday, when I turned the truck off, the noise began. It sounded like an electric motor building up in RPMs, but also very mechanical and loud. Never heard a noise like this before. When I came in the house, my wife said she heard it and thought I had my chain saw out.

I thought maybe the engine was dieseling (rarely does that) so I put on the brake and let out the clutch but the engine was stopped. But the noise continued. I was concerned of a possible fire so started the truck and backed it out of the garage.

I turned off the engine and opened the hood. The noise continued, very loud. I could see nothing. Finally it wound down like a big electric motor and stopped.

I couldn't see anything wrong. After letting it sit, I started it and put it back in the garage. No noise.

Any ideas of what that noise was???

Thanks for your help!