Sacrificial anode.
It is probably sewer gas or something coming up from the drain.
For well water, it is recommended to use a magnesium anode rod instead of aluminum or zinc rods. Magnesium anode rods are better suited for well water with higher levels of minerals, as they are more effective at protecting the water heater tank from corrosion.
A stinky smell in your water heater is often caused by bacteria reacting with the magnesium or aluminum anode rod inside the tank. This reaction can produce a sulfur-like odor. Flushing the tank and replacing the anode rod can help eliminate the smell.
Because the hot water heat transfers into the rod.
The sacrificial anode in a Hot Water Tank takes most of the rusting problems away from the tank itself, hence the word "sacrificial!" When the anode has finished its course of use, you will get rusting happening on the walls of your tank which leads to a wrecked and leaking hot water tank. Manufacturers do NOT want home owners to know about these anodes, because if everyone replaced them there would not be any hot water tanks to replace.
The smell of sulfur in hot water is often caused by bacteria reacting with the magnesium anode rod in your water heater. This reaction produces hydrogen sulfide gas, which has a distinct sulfur smell. Regular maintenance of your water heater, such as flushing it out and replacing the anode rod, can help eliminate this odor.
Follow Some steps for remove magnesium rod in hot water heater.1 Turn off water and fuel supply (gas or electricity) to water heater.2 Partially drain the water heater.3 Locate the anode rod on the water heater; if necessary, unscrew and lift off the cover from the top of the heater.4 Use a ratchet wrench and 1 1/16-inch deep socket to unscrew the anode rod from the heater.5 If the anode rod won't budge, slip a steel pipe onto ratchet wrench handle for extra leverage.6 Lift out and discard the old anode rod.7 Wrap Teflon tape around threads of new anode rod; insert anode rod into water heater and tighten with 1 1/16-inch socket.8 If limited space above the heater prevents the installation of a standard anode rod, install a collapsible type.9 Turn on the water and fuel supply to the water heater.alliedallcityinc.com
Has it been 6 years sense you checked your anode rod. If it is deteriorated this can cause an odor. If the tank is old, this can also cause an odor. Read your manual oh how to check your anode rod, or go online to the MFG website and read it, also look in the troubleshooting section for odor. There could be a answer for cleaning your tank.
All hot water tanks have rust after 4-5 years. If you are a sensible homeowner, drain and clean it every 3-4 years. You first turn OFF the power to tank, now remove the anode rod, ( a large bolt head on top of the tank, attached to what may remain of your rod) . then attach a hose pipe to the bottom fitting, lead it outside or to any drain, turn on the valve by it, turn OFF your inlet valve and open the relief valve so air can enter the H/W tank. Let the whole tank empty -takes about 30 -40 minutes. Next, when empty turn ON the inlet valve and let cold water run through the tank and out the hose, now you will see the sludge coming out. Let this run 5-7 minutes or until water is coming through clean. Now you are done . Let it fill, and fit new anode rod as it fills ( weight of the water helps) then turn on power when full.
turn off water on cold inlet to heater. attach water hose to drain hosebib on front bottom of heater. turn on hosebib, then either open cold water flex line to create a vacuum so the tank will drain. then the anode rod will be the plug style behind the vent or sometimes in front of the vent. if no plug is found the anode rod is attached to the hot nipple usually found on ( a.o smith, bradeford whites and some rheems). remove anode you will notice if its corroded then replace same way anode was removed.
The anode rod in a water heater helps prevent corrosion by attracting corrosive elements in the water, protecting the tank from rust and extending the life of the water heater.
To ensure your home hot water heater lasts long and works efficiently, regularly flush the tank to remove sediment, check and replace the anode rod as needed, insulate the tank to reduce heat loss, and schedule professional maintenance annually.