Low voltage when idling

I have a 99 voyager that has started making a whirring sound this last week. The water pump, serpentine belt, tensioner have all been replace several months ago. The battery light came on for about a minute and then went off the last night. I checked the voltage with the engine running and it measured 12.5, the same as when it is off. When I revved it up the voltage went up to 14.4 volts.

Is this normal? What could be making the whirring sound?

check your charging system, alternator may be on the way out.

I have an '01 T&C that experienced intermittent voltage light. My problem was the belt slipping. It tended to happen in the early morning or when it rained. It turned out the the drain tube from the windshield washer cowling had come loose and was dumping water right on the steering pump. I realize this may not be your problem, but take a close look at the belt.