It sound's like a vacuum leak somewhere the 8th digit of your vin is it a 4 or a 5?I posted a bulletin that is for both those vins let me know what number you have and if the noise is coming from the idle air control valve.
File In Section: 06 - Engine/Propulsion System
Bulletin No: 00-06-04-053
Date: December, 2000
Idle Air Control Valve (IACV) Hissing Noise (Replace the IACV)
1999-2000 Chevrolet Cavalier
1999-2000 Pontiac Sunfire
1999-2000 Chevrolet and GMC S/T Models (510, Sonoma)
with 2.2 L Engine (VINs 4, 5 - RPOS LN2, L43)
Some owners may comment on a hiss noise at closed throttle only. This may sound similar to a belt noise or a power steering pump noise when it is cold. The noise source may be identified by using a hose or a stethoscope and listening in the Idle Air Control Valve (IACV) area. This noise is most noticeable after releasing the throttle at cold idle (105-1200 RPM). The noise is related to the IACV pintle position, typically around 50 counts for cars and 70 counts for trucks.
The condition may be caused when air moves past the shoulder of the IACV pintle. The current profile imparts a disturbance to the airflow, which propagates into the intake manifold. As a result, the manifold walls vibrate causing the noise. To identify if the noise is due to the IACV, connect a Tech 2 to a warm engine and adjust the idle from 1050 RPM to 1200 RPM. The noise will be revealed if the IACV pintle is of the old design.
Replace the IACV with P/N 12482707. Refer to the Engine Controls sub-section of Engine in the appropriate Service Manual for IACV replacement.
Part Number Description Qty
12482707 Idle Air Control Valve 1
Thursday, March 24th, 2011 AT 5:11 PM