Pentair Mastertemp 400 - Rattling on ignition

[texasRay]

This fall, we have had several issues with the pool - a busted (now repaired) pipe, and now we have found that a rat had moved in to the heater and started causing issues. I was able to identify and repair/replace a chewed hose and some wires on the stack flue sensor. The rattling was there prior to replacing the sensor and hose. I have taken off the intake and outflow pipes and made sure they're clear of anything that might be debris (didn't find anything).

Anyway, here's the video:


Any ideas or suggestions on what to do/try next?

 

[Poolbreh]

Edit:sorry I felt like I jumped to conclusions.

so the heater needs to have at least (minimum) 40gpm for the 400kbtu mastertemp to operate correctly.
usually rattling or knocking is due to
A bad thermal regulator
Low flow
Broken bypass
Or build up in exchanger.
If you believe the heater is getting adequate flow, I would pull the thermal regulator first to inspect (small circular cap at manifold).