We moved into a nine year old colonial in the MetroWest area of MA about 2 years ago. After about six months of living there, we began to notice a "rotten eggs" sulfur-like odor when the cold water was turned on in the bathroom faucets, both on the main floor half-bath and upstairs baths as well. The odor only occurs after the faucet has not been used for a couple of hours, and only lasts for 20 seconds or so after initially turning the cold water on (does not happen with hot water). We have had the water extensively tested, and all readings come back normal. However, we had followed the testing service instructions and ran the water for 2 min prior to sample collection. We have noticed a reddish precipitate in the toilet tanks, which I suspect is iron since I can get the slush to move around with a magnet. Our water is from an onsite well, and we don't have any purification or softening equipment. The iron is an obvious culprit here, but I dont understand why the smell would only be transitory when the water is first turned on, and only happens with cold water. Any ideas how or why this is happening would be most appreciated!!