1998 GMC Sierra

I was out in the bush with my truck. Shut the truck off. Was away from the truck for about 5 minutes. Came back and went to start it and it was completely dead. Every was dead. Even the clock and interior lights did not work. There was a bussing-humming sound under the hood in the fuse box vicinity, when the ignition switch was turned on. When the ignition switch was turned to off the buzzing would stop. I was out of the truck with the ignition switch on and all of a sudden the radio came on and I tried to start it and it started and everything worked. I drove it home. A distance of about 10 miles and it worked fine. I tried to start it a couple times since and it worked fine. What do you thing?