Why are you heating the hot water? I usually heat cold water =). Larryetc is probably right that its just water in the air.
In case its not, when was the last time you sanitized the (cold) water heater? Might be time to put in a bleach solution, let it sit and then flush the (cold) water heater a couple times. Any funky stuff won't survive the bleach and the flush will rinse out any other debris that might be causing it.