Q: I have a pentair 400 heater. It has been making a knocking sound like their is a rock hitting . I know it is not air. It still heats but when it starts it knock I turn it off. Some one subjected I take off the cover take it could be a rock in the fan area.
A:
The knocking sound could be not enough water flowing through the heater, possible water chemistry issues like calcium buildup in the pipes, or a valve issue. It could be serious enough to need get a pool guy out to take a look.
Good luck!