My refrigerator is making a knocking noise, what is causing it?

It could be at the least, the fan in the freezer hitting ice, the fan blowing on the compressor out of alignment or worst case scenario, the compressor is starting to fail. Where is the sound coming from?

XenoSapien

It sounds like it's coming form the back at the top.

That is where your evap fan is it might have came loose or the back wall out of alighnment or as xenosapian stated it could have ice building up in there and hitting the fan