Refrigerator making loud clunking sound

A large clunking sound could be many things, Since Refridgeration equipment is a licensed trade we can look at a few things. But with the available information it is hard to provide a proper answer. Two long for a comment so here are a few things to check.

Blew out the coils, is the fan clean and functional if it has one.
With out a proper air flow on the condensing coils the heat exchange is limited. I ask about the fan because it can be the sound hitting something or clunking Next I would look in the freezer section is it a frost free model? Again is the fan working, coils iced up? Usually with frost free you have to remove covers or defrost if it works for a while after defrosting it could be the fan or the defrost timer/ heater to thaw the coils. If the above all check out we are to a pressure check. Most refrigerators do not have a service port so one has to be added to check the charge and if needed recharge the system.. there are mechanical port / valves that can be added for under 20$ but these are questionable in my opinion but may be an option for a less expensive repair if the system just needs a recharge.