Water heater mineral deposits! We have such hard water that our water heater starts making a banging noise when there’s lots of build-up. Our plumber installed a brass faucet, taught me how to turn off our water heater, connect an old hose and drain it down the driveway every 2-3 years (tried to recycle the water, but the hot water burns the lawn and plants). When we last needed to replace our water heater I considered a tankless but discovered I would have to descale+clean the filter every 2 years anyway. Plus would lose my 50 gallon back-up water to boot. I keep a small metal kitchen strainer in a ziplock bag inside the water heater closet to strain the deposit if we ever need the water.