Quick fix with Blowdryer
I have an LG fridge (side-by-side model) that was making the same noise: like something hitting the fan. Instead of letting the fridge defrost, I got a blow dryer and aimed it at the vents in the back for few minutes. That did the trick - no more noise.
Thanks for the insight that it was likely ice build-up around the fan - I would not have thought of the blow dryer otherwise.