We installed this tankless unit about 2.5 months ago and have been having trouble with a deep rumble sound that reverberates throughout the house. It only occurs at random times and lasts only a couple of seconds. We have 3" pipe - exhaust and intake next to each other going horizontal from the unit and then vertical in the existing 8" flue going up about 15' through the roof. On the roof the exhaust pipe goes straight up with a cap on it, while the intake is 3' lower with an elbow pointing sideways. We live at about 6500' and have adjusted the dip switches for altitude. Still getting these random rumbles. Does anyone have other ideas about what else we can try to eliminate this problem.