At wits end for clicking noise/won't start for 1999 GMC Sierra

Group-

Been awhile, I've been on deployment on the USS Enterprise in the gulf since last summer. No fun. I have a 1999 GMC Sierra Z71 ext cab 5.3l. It runs fine, but one thing really gets my goat. I disconnected the battery when I replaced the heater blower motor and resistor (it was snapped in half!). After I reconnected the battery, I started up, everything great. I then killed it and then went to bed. I didn't bother to lock the truck. When I got up the next morning and went to leave, I opened the door to the truck. I heard a constant clicking noise, which lasted for about thirty seconds to a minute. I could not start the truck. Even after the noise went away, I couldn't start it. I got out, then got back in. This time, on a whim, I pressed the unlock button on the door. The clicking noise went away, I turned the key, it started, and away I went. Hmm.... So here's the deal: if I unlock the truck by the key fob or by the button on the door, the clicking will stop. Pressing lock will not work--only unlock. I'll admit it's not a serious problem, but it drives me nuts. Any ideas? Any advice, no matter how far-fetched it seems, will be appreciated. Thanks for your patience.

-Guido

Sir-

It is just the factory theft deterrent system. Thank you.

-Guido

Look under the driver's side dash for an aftermarket alarm. We used to use them at the dealership where I work after getting a bunch of cars stolen and then just disable it after the sale but not completely remove the alarm. If there is one there just remove it and hook the factory wires back up and you should be good. The only wire I remember being cut in half is a yellow wire for the starter and the rest were just taped into. Hope that helps.