We recently purchased a cabin and discovered that there's an unpleasant odor at all sinks when running the hot water. There's a kitchen and bathroom sink on first floor and a bathroom sink on second floor. We didn't test the shower or bathtub. We don't smell the odor when cold water is running. Water source is a drilled well to a 50 gallon hot water heater. The cabin has a septic system.

Any ideas on cause of the odor?
thanks