Water Heater makes tone.

Some times my water heater gives out a loud tone, when it's heating up, sometimes not always. I've seen posts about banging and thumping but not about a tone. Last night it went off and it was so loud it woke me up.
It's about four years old. I have really bad water here so, every spring and fall, I take the bottom element out and clean the tank with a shopvac and hose to get all the sediment out. Sometimes I replace the element sometimes not. This last time about a month ago, I decided I would go ahead and replace the top element too since I can't remember if or when I've ever done it. Now a month later this sometimes tone.
Anyone know why? Thanks

sometimes when I fill a heater for the first time the water lines (assuming they are flexes/corrugated) make a high-pitched tone. Usually subsides after the tank is full. Maybe the water lines? Also, I once had a problem with a water pressure regulator near the water heater. It had debris in it and made a noise when the water would run through the piping. It turned out when the main water to the house was shut off to service the heater, some debris from the shut-off valve was knocked loose and gummed up the regulator. hope this helps.