Help Finding Electrical Gremlin - Mystery Sound (video link)

Hey everyone! Long-time lurker, first time poster. This forum has been immeasurably helpful to me over the years that I've owned my Tundra (over 14 years!), and I'm very thankful to all the very knowledgeable folks here. I've now hit an issue that I can't diagnose by scouring the forums as a lurker, so I decided to make an account and ask for some help directly.

I have been dealing with "battery issues" for several months now. The truck (01 AC SR5 4.7l V8) has been driven regularly, including the day before yesterday. Came out yesterday morning to a dead battery. Charged the battery back up and reconnected the battery cables only to hear this sound: (video link). I have heard this sound once or twice before while the truck was cold (i.e. hadn't been running for 8-12 hours +) before being greeted by a dead battery. I doubt the battery is the problem - it's only a year and a half old, and the issue has been going on for the better part of a year.

I know there has been something draining the battery these past few months, but I haven't been able to diagnose the electrical issue. Any ideas on what it could be? I just received all the parts for the Timing Belt/WP job that I plan to start soon, so I'd love to tackle this problem while I'm at it. Any help is appreciated!