Diagnosis
This is fan noise. The most common cause is dust accumulation on the intake filter or inside the projector, forcing the fan to work harder. A less common cause is a failing fan bearing.
How to Fix It
First, power off and unplug the projector. Let it cool for 30 minutes. Remove the air filter cover on the bottom. Take out the foam filter and clean it with compressed air or by gently washing it with water (let it dry completely). Use compressed air to blow dust out of the internal intake vents. Reinstall the dry filter and cover.
If That Didn't Work
The internal fan itself may be failing. The projector has an internal temperature sensor. If the fan RPM is insufficient, the projector will overheat and may shut down. This requires opening the chassis to inspect, which voids the warranty. Contact ViewSonic support for a service procedure.
Heads Up
Never operate the projector without the air filter installed. This drastically accelerates dust ingestion and will cause permanent overheating damage to the DLP chip and lamp.