Vehicle noise

I have a 1997 Chevrolet 3500 crew cab pickup with 115,000 miles. The last couple of days after starting the engine I hear a whirring noise from somewhere around the firewall in the inside. I have listened under the hood but am unable to hear it from the outside. The whirring coinsides with the RPM of the engine whether the transmission is in gear or in neutral. After heating up to driving temperature the noise seems to subside. Any help will be appreciated. Thanks

fluid flowing/not through the heater core?

Ah, your problem sounds like a fan de-clutcher. Easiest way, is just lift the hood, grab the fan and if it doesn’t turn easily, there’s your demon.