Low-frequency noise and rumble problems can occur in HVAC systems due to problems with the fan, the duct sytem, the fan/duct interface, and even the control system. This discussion deals with fan related problems only. Items that will be discussed include: the importance of fan-type selection (centrifugal, axial, plenum, etc.), selecting the right fan size for the required duty, the importance of aerodynamic conditions at the fan inlet and outlet, and the problems associated with VAV control mechanisms. The discussion will also address the importance of proper vibration isolation and how to prevent common problems. Specific project examples will be included.