[5pickr]

Ok- I have been lurking for a couple weeks and have reached the help me point. I have a Frigidaire SxS Professional Model# PHS69EHSS2. The unit is approximately 5 years old. The symptoms are:

-Loud, high frequency knocking noise sounding like its coming from the upper freezer compartment, almost like its the ice maker, but I suspect its not
-Coils frozen in lower freezer compartment
-Freezer and fridge warm to 57 degrees

It did this two weeks ago. After checking this forum for advice, I replaced the defrost heater and the clip-on thermostat. It has worked normally for a week. Last night, I heard the knocking noise again for about 3 minutes, then it quit. This morning, freezer and fridge were showing 57 degrees, and ice was starting to melt. A quick check of the coils and no apparent ice on them.
So, is it worth replacing the motherboard, which seems to be the next most common replacement, or is this, as I suspect, a closed system problem? The knocking noise is "hard" enough that it does not sound like a fan blade ticking on ice or anything like that. It sounds like a small air compressor kicking on. A new board is another $145. Should I put that towards a new unit?

Thanks in advance for any help here...

JT