Every water odor has a short list of causes, and the fastest way to narrow it is to note where and when you smell it. Hot tap only, cold tap only, one fixture or the whole house, straight from the tap or only in a filled glass: those four observations do more diagnostic work than any test kit you can buy.
Run the four checks before anything else
First, hot only or cold too. Second, one fixture or every fixture. Third, does the smell survive in a glass carried to another room, which separates water odor from drain odor. Fourth, does it fade after running the tap for a couple of minutes, which points at something sitting in the plumbing rather than in the supply. Write the answers down before you call anyone, because they are what a competent technician will ask first.
Rotten egg and sulfur smells
Hydrogen sulfide smells of rotten eggs. If it appears on the hot side only, suspect a reaction between the water heater anode rod and sulfate reducing bacteria in the tank, which is a heater problem rather than a supply problem. If both hot and cold smell, the gas is arriving in the water, which is common on wells and needs treatment at the point of entry, typically oxidation followed by filtration or, in mild cases, aeration.
Musty, earthy, chlorine and metallic
Musty or earthy smells usually come from harmless organisms in a surface water source or from biofilm in the plumbing, and carbon filtration handles them well. A chlorine or swimming pool smell is the disinfectant your utility uses, which carbon also handles. Metallic odor with a taste to match points at iron, copper or an old galvanized line, and it is worth checking whether it is worse first thing in the morning after the water has stood.
Use your water report before you buy equipment
If you are on a public supply, every community water system publishes an annual Consumer Confidence Report and the EPA maintains a finder for them. That report tells you whether your utility uses chlorine or chloramine, which decides the carbon media you need, because chloramine requires far more contact time. On a well, nobody publishes anything and you order your own panel: our well water testing guide sets out what to include.
Questions people ask about water odor
Why does only my hot water smell of rotten eggs?
That pattern points at the water heater, usually a reaction between the sacrificial anode rod and sulfate reducing bacteria in the tank rather than anything in the incoming supply.
Is smelly water dangerous to drink?
Odor is usually an aesthetic problem rather than a health one, but it can also be the first sign of a change in a well, so a sudden new smell on a private supply is worth testing.
Will a carbon filter remove water odor?
Carbon handles chlorine, musty odors and many organic smells well. It is much less effective against hydrogen sulfide at high levels, which needs oxidation and filtration instead.