[izzy_Britton]

sounds to me like it could be one of 3 things, first question is: does your horn work properly?

it sounds to me like your truck has/had an alarm, maybe aftermarket and when you open the door the lights are flashing as i would assume your horn doesnt work or it might start blaring at you too. usually turning the key to run will kill the alarm.

second question: do you unlock your truck with the OEM key fob or do you use the key in the door? if you use the OEM key fob, try using the key. it just sounds like the alarm is trying to go off.

second thought is that you either have a bad relay or that someone along the way has installed the wrong relay or even a LED lamp relay in the spot that controls your dome/parking light and because it is failing it is sending a short to the other half of the relay causing what you have going on.

3rd thought is that the GEM is failing for one reason or another. the GEM is what usually controls your door dinger when the door is open and the key is in the ignition, also your flashers, your windshield wipers, your seat belt or airbag safety warning lights or buzzers.

I am thinking most likely it is the alarm just because when you turn the key it quits. like i mentioned, usually doing something like pressing the lock button on the inside door panel, closing the door, then opening the door using the key and starting the truck will do a soft reset of the alarm. if the truck is new to you i would bet that someone along the way someone installed the aftermarket alarm and perhaps you do not have proper key fob for that alarm.

let me know. I am curious now.