Diagnosis
Something is interfering with the fan blade. Common culpits are a loose piece of debris (like a plastic shipping tag) that got sucked in, or the fan blade itself is hitting its housing because a mount is broken.
How to Fix It
Turn the unit off and unplug it. First, remove the air filter (see the other question on that) and look inside the intake area with a flashlight. See if you can spot any foreign objects. Use needle-nose pliers to carefully remove anything you see. Also, gently try to wiggle the fan blade (usually visible behind the filter area) to see if it's loose on its shaft.
If That Didn't Work
The noise could be coming from the exhaust fan on the other side, or from a failing fan motor bearing. Those are harder to get to and usually mean the motor assembly needs replacement.
Heads Up
If the noise started right after you moved or tilted the unit, it's almost certainly a piece of the internal drain pan or a loose screw rolling around inside. You'll likely have to open the main cabinet to find it, which voids any warranty.