1. Replacing elements 2 or 3 times in 4 months is extreme. They last for YEARS
2. Since you have replaced the elements and it appears, NO noise until you did this indicates your elements are probably not OEM quality AND correct Length. Where did you get your elements? Local HD or Lowes/Ace Hardware? If so, odds are they are LONGER than required and maybe touching the back of the tank interior pick up tube and that is causing the noise on Heat Cycle.
3. You need to remove the Drain plug and do a complete flush of the inner tank.
4. I would get the Atwood or Suburban(whichever brand you have) OEM element and compare to the ones you have installed. Doug