I believe the whirring is the ABS unit, under the hood, testing itself. The is TSB NTB10-077:
ABS SELF CHECK NOISE
APPLIED VEHICLES: All Nissan Vehicles with ABS or ABS/VDC
SERVICE INFORMATION
If you identify:
• Clicking, knocking, clunking, buzzing, or thumping noise coming from the engine compartment area; and
• The noise occurs only once during the first few seconds of driving at approximately 16 MPH; and
• The noise occurs only once per ignition cycle (ignition OFF > ON) and does not occur again until the ignition is cycled.
NOTE: The noise may be louder after the vehicle has not been operated for a prolonged period of time.
The above condition is normal and does not need repair.
Each time the ignition is turned on and the vehicle is driven at about 16 mph, the ABS/VDC system performs a “Self-Check” to confirm components of the ABS/VDC system are operating correctly.
This “Self-Check” function creates the noise described above.
If the brakes are being applied when the “Self-Check” occurs, a vibration may be felt in the brake pedal and an increased level of noise may be noticed.
NOTE:
• Refer customers to the Owners Manual, Section 5, Starting and driving, Brake system on the Anti-Lock Braking System (ABS). The “Self-test feature” section provides additional explanation.
• The speeds described above refer specifically to Versa, Sentra, Altima, and Maxima. However, any vehicle fitted with ABS or ABS/VDC will have a “Self-Check” which is triggered in similar circumstances.
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