Whats wrong with my car? (I included a detailed description)

I have a 97 pathfinder. I noticed that it was leaking oil, so I changed the valve covers. Now, the RPM needle goes crazy, the airbag light is flashing, and just today after getting home from work I went to leave again, and no power when I turned the key. I popped the hood, checked the battery cables, and nothing. I let it sit for about 10 min or so, and when I went back out I turned the headlight switch on and started checking the connection again. About 2-3min later I was going into the house and I heard the power come back on(bc I had left the key in the ingition) I started the vehicle and checked the altenator by removing the neg. Batt. Cable(alt is good). Upon turning the vehicle off, I could hear a loud rapid ticking/buzzing sound coming from under the hood. I turned it back on to see if it would start again and it sounded louder than when I turned it off. I haven't found anything about it on the net, please help