99 Chevy Astro with 93000+ miles. Only recent problem (one week ago) was left lights on for 4hrs on a new battery and almost completely drained battery, then, vehicle cranked without a jump. This AM started van to go to work and there was a definite “whirring noise” from under the hood. Popped the hood and sounded like coming from the brake master cylinder area vs. AC compressor area-hard to tell though. AC has not worked for 2 yrs. Noise increased in proportion to revving of engine. Likewise, noise continued while driving, although seemed to not be as loud when engine warmed up. Temp outside 34. Fluid levels OK. Any idea where the noise is coming from? Also not sure if related or a concern: brake pedal has a slight “stick” to it when initially pressing the pedal, for ~ last 6 mths. give or take Brakes seem to work fine.
Carefully - use a length of any kind of flexible tubing/hose as a makeshift stethoscope. Its a good way to pinpoint a noise.
Since you recently beat the heck out of the alternator (first from the old battery, then by making it recharge the new one) do pay special attention to it.
My guess would be the alternator being stressed attempting to recharge the dead battery. I install a battery disconnect switch on a 2000 Blazer with 121k miles. It was making the same noise and changed with engine rpm. I removed the switch and the noise went away. The switch did not have a good connection closed.
I suggest fully charging the battery with a stand-alone battery charger.