Any Chance you could upload an audio file recording, perhaps a video with the noise to youtube and then link it here? Someone may be able to give more information based on the actual sound.
The two knocking noises from a fridge that dealt with the in the past where, a) the ice machine noise you describe, and b) an off balance fan due to ice accumulation on the blades.
a) isn't hard to diagnosis, as has already been suggested turning the ice maker off should stop the noise if is actually the cause. It could be a particularly stubborn ice cube or it could be that one of the gears (likely plastic/nylon) has worn to the point where it is slipping under load.
b) in my case it was a know defect with the model and there was a revised part assembly with a second bearing that corrected the issue.
If you can track the problem down to a specific part, not just something in the ice maker, a specific gear for example, site like repairclininc.com and ereplacementparts.com will likely be able to supply you the part needed.