Diagnosis
That's usually the fan. Either the blower wheel has collected a bunch of dust and is out of balance, or something (like a stray Lego) has gotten inside and is hitting the blades.
How to Fix It
Unplug it. Remove the bucket. Look at the rear grille—that's where the fan pulls air in. Use a flashlight to peer inside and see if you can spot any obvious debris. You can try using a vacuum hose with a narrow attachment to carefully suck out dust and hair from the fan area. If the noise is more of a vibration, check that the unit is sitting perfectly level on all four feet.
If That Didn't Work
The compressor might be making the noise, which is a bigger deal. If the sound is a deep, metallic grind from the lower half of the unit (not the top fan area), and it happens shortly after startup, it could be a failing compressor. That's a repair-or-replace scenario.
Heads Up
Never try to spin the fan blade with your fingers or a tool while it's plugged in, even if it's off. You could damage the motor or the blade itself.