Water Heater Knocking

[SD303]

I have a 40 gal Gas Direct Vent "Powerflex - American Water Heater".
About 10 minutes after it turns on, a knocking noise comes from inside. The best way to explain it would be that it sounds like rocks are moving around inside. It last until it turns off. I have no signs of leaking and have no problems getting hot water in showers etc. I drained the heater and noticed the water was normal color. The heater is about 8 years old and this problem started about 2 weeks ago. Any suggestion?

[majakdragon]

SD303, Welcome to the DIY Forums.
I would say the problem is sediment on the bottom of the tank. When the unit turns on, the sediment is actually "frying". You mentioned that you drained it. Did you ALSO flush it? This needs to be done also. The cold water coming in through the dip tube will get the sediment moving around so it can exit through the drain. I have included a link with proper procedure to do this. Just click on the link. Good luck.

http://forum.doityourself.com/showthread.php?t=134483