One common cause of noise in rotary airlocks is rotating blades making contact with the stationary housing. Each airlock is designed so rotor/housing clearances can operate without contact at some maximum temperature. If this temperature is exceeded, the thermal expansion of the rotor can exceed that of the housing, at some point leading to metal-to-metal contact and noise. If the temperature can be reduced to the design point, this is the easiest answer. Another solution is to have a qualified shop machine the rotor diameter/length for this higher temperature and eliminate this noise and a potentially serious mechanical problem. Contact may also happen for process temperature conditions that are changing (e.g., start-up), where the external housing may not heat up as quickly as the rotor. The simplest solution is often to reduce the rate of heat up to avoid the rotor-to-housing contact. Secondly, some materials (e.g., lime, plastics, sugar) under certain conditions (temperature, shear stress, moisture) can buildup on the inner dimensions of the airlock housing and can make noise as the blades shear through this hardened thin material layer. If material buildup is causing the problem, a modification to your process conditions may be recommended. Contact your process engineer or airlock supplier to assist you in these reviews.