The truck has a bad recirculation door motor, with ignition off the motor is trying to find home position.
Quick fix is to unplug the motor, to replace the motor GM says you need to roll the dash back.
I have replaced those motors without rolling the dash by using a 1/4 inch air rachet and a 5.5mm snap-on flexsocket and guessing where one of the screwhead is because you can't see it.
William
97 Chevy Tahoe 2Dr 6.5 Kennedy Exhaust Pipes w/Homemade Resonator and Flowmaster Muffler, Turbo Tech intercooler and Kennedy/Westers computer
01 Sunline 2363 Travel Trailer