Change your anode!
Later today I'll be changing out the anode in my water heater and thought...hmmm I wonder how many people do this sort of thing?
It's a good thing to do every few years, especially if you have a finished basement as I do. Also water heaters are pretty expensive, and this will keep it lasting longer. The hardest part is getting that anode out, but if you have the right tools, and a cheater bar, it shouldn't be that hard to do.
I ordered my anode through Amazon and it was about $50 bucks. There are cheaper one's made of aluminum.
Once you replace it, write on the water heater with marker the date it should be replaced.
The video from This old house explains it very well. That cut-out is pretty gross though.