Inspected water heater. Getting a slight smell of sulfur from around the vent hood when burner is running. Flame on the burner looks good and combustion chamber is clean. It appears to be drafting just fine, also did a flame test which looked good. Used sniffer to test for a gas leak but didn’t find any. Customer has a carbon monoxide detector next to the water heater that isn’t going off even when placing it next to the vent hood. At this point there’s no explanation for the smell but based on everything mentioned above it seems safe to operate.