I work for a plumbing wholesale company, so I have delt with this problem before. There are two things that could be causing this problem.
1. Typical the smell is due to a bacteria that enters the water system due to construction or a break in the line. The simplest treatment available is the shock-chlorination of the system. This is a surface treatment, and often requires repeated trials in heavily infected systems. The chlorination of a system requires that you follow each step explicitly to avoid an un-treated portion of the piping system from reinfecting another part. Longer lasting solutions include chlorination or aeration of the water supply.
2. (*Most Common) The anode rod can react to a bacteria in the water. Change this to a Magnesium anode rod.