I am trying to remember what all of the different lights and buzzers were supposed to warn the driver about some sort of condition that needed attention.
Every emergency exit door, hatch, and window had some sort of alarm. The roof hatches in particular are sometimes really hard to get the buzzer to quit.
Low oil pressure should stop buzzing as soon as the oil pressure goes over 5 PSI.
Low air pressure should stop buzzing as soon as the air pressure goes over at least 60 PSI and it may stay buzzing up to 90 PSI.
Some buses had a headlight buzzer if you didn't turn the headlights off when you turned off the key. Others had a buzzer that would only go out after the headlights were turned on whenever the key was in the run position.
Some buses had seat belt warning buzzers the remind the driver to put on driver's seat belt.
Some buses had a buzzer for low fuel.
You are going to have to start going through the electrical panel and start turning circuits on and off so that you can find which circuit is powering the buzzer. Once you find which circuit it is on you should have a better idea as to what the buzzer is telling you to check.
Good luck and make sure you post what the buzzer was reminding you to do!