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.
FIX 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.