Check hybrid battery light on

Hi guys, so my gen 3 2010 prius with 130k miles on the engine, it's "check hybrid battery system" light just turned on--when i was in the middle of an uber as a matter of fact :[ The engine still runs and I finished the ride before calling toyota to schedule a diagnostic and then returning home to come post this forum. For background I have been hearing some strange sounds, particularly a humming/whirring sound when the gas engine kicks in. Additionally there have been times when i've been at a stoplight wondering why the gas engine hadn't shut down. Anways I have a few questions:

-CA 10 year 150k mile hybrid warranty; this is applicable in my case, so I shouldn't be charged for the replacement correct?
-will the diagnostic test cost me anything? I'm pretty broke
-will they, upon diagnosing and potentially just fiddling around here and there with mechanical things (potentially turning off the system warning), try to avoid replacing my battery? That is, will they find a temporary solution to my problem that doesn't replace my hybrid battery--**like some alternative because it's not completely dead--and and potentially put me outside coverage?
-also can anyone address that strange humming/whirring sound that occurs when the engine kicks on from the hybrid standby mode (like at a traffic light)