I have a sta-rite heater that's probably original so around 12 - 13 years. It makes a pretty loud knocking noise a minute or two after firing up. It almost sounds like something is loose and banging around inside the lower part of the heater. The heater eventually cycles off.
I replaced both the thermal regulator and bypass valve, but it still knocks. I think after doing some research that lime-scale buildup is probably causing some internal issues. I've had some really hard water for too long and just recently replaced it.
I think I could follow this youtube video https://youtu.be/w5fEnagoRbU and probably get it fixed. That is my heater and the video is pretty detailed.
However, I'm not 100% sure if this is the best route and I would like some advice on whether I should just replace the heater instead.