Water quality issues
Presence of sand/sediment
Do you have a water pressure problem?
Do you have a water treatment system?
Is the smell noticeable only in hot water, or in both cold and hot water?
First of all, verify the pressure tank. Try to gently shake the top of the pressure tank (push the top side to create a back and forth movement). Do you hear water moving? Bubbles or waves?
Is your water non-potable (fecal contamination by E. coli or enterococcus)? Or is it nonconforming (atypical bacteria or total coliforms contamination)?