Welcome to TFP.
The description of knocking in the heater and LO code is indicative of low water flow. Low water flow is usually caused by a clogged filter. Filters often clog due to unseen algae in the water.
The fact that your heater ran for a while after you put new filters in further confirms you probably have algae that clogged the new filters in a few hours.
The knocking is caused by water boiling within the heater due to the low flow. Then as the filter clogs more the flow gets to where it will not engage the pressure switch and you get the LO code.
Yeah, you say your pool water is clear. Sorry, we see this alot. Clear pool water can have algae.
I suspect you don't have your own test kit for your pool water. You need to get a Taylor K-2006C or TF-100
Test Kits Pool store test will not be adequate for what you need to do.
We treat algae problems with the
SLAM Process and you can get familiar with that. Also read
ABCs of Pool Water Chemistry
But the first step is to get a good test kit and post the test results and do a
Overnight Chlorine Loss Test to confirm if you have an algae problem as I suspect.
We can work you through this if you want and likely no further disassembly of your heater is needed.
@Pool Clown @ps0303 @swamprat69 thoughts?