I need some opinions. Two weeks ago I noticed brown/yellow water on the hot side of the tap (the cold side was clear). So, I flushed the water heater several times and it cleared up. Today, all of a sudden, it turned brown again. How can it reappear so quickly? Is this a sign the heater is rusting inside?? It's 10 yrs old, but I flush it once per year. The anode hasn't been replaced. It's $600 for a new one so I don't want to replace it unless totally necessary. What do you think is causing this?