One of the units developed a clicking sound when it oscillated. I took it back and exchanged it for another one. After a few minutes it developed the same clicking problem. I took it back and decided to give it one last try. Unfortunately, that unit developed the same problem after only a few minutes of use. I took it back, got my money back and went to Sears who also sells the Bionaire products. They were out of the AP1500-U PERMAtec Ionizing Air Cleaner so I purchased the BAP2000-U PERMANENT FILTER AIR CLEANER W-GALILEO which was considerably more money. This unit also has the same clicking problem as the AP1500-U. I contacted Bionaire customer service and was told that they did send out a bad batch and they suggested that I send the unit back to them for a replacement at my expense. Let me add that they said that there would be no guarantee that a different unit would not exhibit the same problem. My suggestion is to stay clear of these units until they determine what is causing the defect and seek out another brand of air purifier.