Diagnosis
You're likely hearing the laser diode driver or the color wheel spinning at high speed. A faint, steady whir is normal, but a pronounced, high-pitched buzz or whine that changes with picture brightness is not.
How to Fix It
First, identify when the noise happens. Turn off all audio and listen. 1) If the noise is constant once powered on, regardless of the image, it's likely a fan bearing or the color wheel assembly. 2) If the noise gets louder or higher-pitched when a bright white or colorful scene is on screen, that's the laser driver working harder. For the second case, go into the projector's menu: Settings > Picture > Advanced Settings. Look for a setting called "Dynamic Contrast," "Laser Output," or "ECO Mode." Turn Dynamic Contrast OFF and set the Laser Output or Picture Mode to "ECO" or "Cinema." This reduces the peak laser power and can often eliminate that specific electronic whine.
If That Didn't Work
If the noise is a grinding or rattling buzz, it could be a foreign object like a tiny piece of foam from the packaging stuck in a fan. You'll need to carefully inspect the intake and exhaust vents (with the unit off and unplugged) and use a can of compressed air to blow out any debris. Don't shake or tilt the projector aggressively.
Heads Up
Never try to open the sealed optical engine yourself to get at the color wheel. That's a laser safety hazard and will void your warranty. If the noise is mechanical and the compressed air trick didn't help, it's a job for LG service.