What you're probably noticing
The shower doesn't hold heat like it used to. You used to get fifteen minutes before it turned lukewarm, now it's eight or nine. There might be a low rumbling or popping sound coming from the water heater closet or garage corner, especially right after it kicks on. Maybe the water ran a little cloudy or had a faint mineral taste the last time you filled a pot. None of this is dramatic. That's exactly why it gets ignored until the tank fails on a Saturday morning.
Why this shows up here in Nixa
Nixa and the rest of southwest Missouri sit on water that's harder than a lot of the country realizes. The limestone and dolomite bedrock under the Ozarks feeds a lot of dissolved calcium and magnesium into both well water and the municipal supply. That mineral load doesn't disappear when it goes into your water heater tank. It settles out as sediment on the bottom, especially once the water gets heated repeatedly.
A lot of housing stock in and around Nixa dates from the growth years of the 1990s through the 2010s, which means a good chunk of water heaters installed then are now well past the point where sediment has had time to build up undisturbed. If nobody's flushed the tank in that stretch, and most people haven't, there's likely a layer of scale sitting at the bottom right now.
More on this from Sediment, Rumbling And A Shorter Hot Shower.
What the rumbling actually means
That popping or rumbling sound is water trapped under a layer of sediment, boiling and pushing bubbles up through the mineral crust. It's not the tank itself failing, but it is the heating element or burner working harder than it should to get the same result. Over time this wastes energy and, more importantly, it accelerates wear on the tank lining and the element or burner assembly.
A shorter hot shower is the practical result. Sediment insulates the water from the heat source, so less of the tank's actual volume is getting heated efficiently. You're paying for a fifty-gallon tank but effectively getting the performance of a much smaller one.
What you can check yourself
Look at the drain valve near the bottom of the tank. If you've never flushed it, or don't remember the last time, that's worth doing before winter arrives. Turn off power or gas to the unit, connect a hose to the drain valve, run it outside or to a floor drain, and let several gallons run out. If what comes out looks gritty, cloudy, or has visible sediment, that confirms what the rumbling was telling you.
Also check the age of the unit. There's usually a manufacture date on a sticker near the top, sometimes coded into the serial number. Water heaters in this region, given the hard water, often don't make it much past ten to twelve years even with reasonable care. If yours is already there, sediment is compounding a problem that's going to end in replacement regardless.
Check the temperature and pressure relief valve while you're at it. Lift the lever briefly and make sure water discharges and then stops. If it doesn't discharge, or it won't stop dripping afterward, that's a separate issue worth noting.
When it's time to call someone
If the drain valve won't open, or opens and then clogs with sediment partway through, stop. Forcing it usually just breaks the valve, and now you've got a leaking water heater instead of a slow one. That's a job for someone with the right tools to clear or replace the valve without draining the whole system in an uncontrolled way.
If the rumbling is loud and constant rather than occasional, or if you see rust-colored water, moisture pooling at the base of the tank, or the unit is more than ten years old and struggling, it's worth having someone look at whether flushing will even help at that point versus recommending replacement. Sediment that's been baking onto the tank floor for a decade in hard Ozarks water doesn't always come out with a simple flush, and pushing water heater repairs past their useful life usually costs more in energy and water damage risk than replacing the unit would.
Winter is also a bad time to discover a tank is failing. Cold snaps in this part of Missouri can drop overnight lows well below freezing, and a water heater that quits in January is a different kind of emergency than one that quits in June. If your unit is showing sediment problems now, in the low-key months, get it addressed before the ground freezes and everyone else is calling for the same reason.