The usual small from a water heater is rotten eggs (hydrogen sulfide), caused by a bacteria that can thrive in hot water in dark places. A 4-6 hour bleach solution will cure that, but you also need to sanitize the hot water lines or it will come right back.
You don't need pure vinegar for a tank soak - 10- 20% solution should be plenty.
I'd replace the anode too, "just in case".
If none of those do it, I'm fresh out of ideas.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL