On Sun, 7 May 2017 20:13:18 -0500, "Snags backup"
wrote:

and every time we use much hot water we can hear the heating elements "sing"
, apparently because the water is boiling off the elements . We also have
very hard water , and I'm wondering if because of the low pressure and the
"singing" we've gotten a buildup of mineral deposits on the elements . I
think that may insulate them from heat transfer and has caused them to
overheat and burn out .

The hard water minerals most/may likely settle to the bottom of the
tank, harden and "cake" up. What you may be hearing, when water is
heating is the minerals start to "peculate" (boiling water) from the
tank bottom. Sort of a 'rumble' as the minerals break from the bottom
an circulate again. Hearing my last tank I said to myself -- self --
this WH is on its last leg. Got a new one installed on Super Bowl
Sunday. Lucky it sprang a leak the moment I walked into the garage and
could shut the water supply off at the ball valve. Good luck Minerals
are hard on water heaters here though I have a softener.