Charging System

1997 Dodge 3/4ton 5.9L Cummins Turbo Diesel, 2 wheel drive, extended cab long wheel base, NV4500 5-speed. About 250,000 miles. My truck is having problems after I start it up, while the truck is running my charging guage is reading towards the discharging side, then all of a sudden it will charge, and will keep going on like that for a while. There is something ticking under the hood that makes it do that, it's almost like leaving your car or truck to idle witht he air conditiion on, but it's not on in my truck. I went down to get my alternator checked about a month ago and it came out good and both batteries too. I don't know what else to think of.