I thought it was the alternator, but I had it checked out at O’Reillys and they said it was fine. But my battery water was extremely low. I replaced the battery. I can smell a little bit of a burning smell once in a while, my dash lights flicker bright and then dim, and sometimes they turn off all together however when the dash lights go out, the mileage, the airbags off symbol, and the indicator that the brights are on all remain lit. No power to the radio. when I turn the vehicle off, the cabin light goes off after timing out like normal and then a whirring and/or buzzing noise, and the cabin light turns itself back on for like 15 seconds, then turns itself back off. Then wait 5 mins and a shorter buzz sound and cabin light turns on, quick buzz sound again and light goes off again. Light is on for a shorter duration the second time. There are two or three different buzzing or whirring sounds that occur coming from the dash once the engine is shut off. While driving the dash lights will flicker sometimes and sometimes they won’t. I believe it started with the radio, it used to turn off on its own and then turn back on while driving. This happened randomly for years with no other issues. But now it doesn’t turn on at all. I don’t believe it’s the PCU because I replaced that a couple years ago and everything was fine. Checked fuses-found nothing. Any ideas?

 

Alternator doesn't break and it should last the lifetime of your truck. If something goes bad it's the brushes which normally lasts 5 years and require routine replacement. The brushes sit between your alternator and your voltage regulator. Depends on the car make/model, some brushes come with a new voltage regulator if it's replaceable, or attached to the alternator and replaceable.

To narrow down the possibilities you need to remember when it's happening, engine off key out or key in ON/RUN position or engine is running, find out which of the 3. When engine is running, it's all alternator. When engine off key out it's battery with always hot 12v feed (i.e your horn your hazard), when key in ON/RUN position (i.e. your radio your turn signal stalk) it's also your battery.
And keep in mind you do not need an alternator to start your car, you also can disconnect your battery ground cable once your car is started for testing purpose in short period of time like few minutes. Based on this, you know what to test now. If engine running or key in ON/RUN position doesn't work, you need to check on your ground wires, and preferably just clean all major ground points to begin with when you have major electrical issues.

 

Voltage regulators in alternators fail all the time - especially when cheaper parts are used.