Bionaire Air purifierProduct deails Using your ionizer may result in an occasional popping or cracking sound. This is a normal sound, generally caused by particles of dust that are interrupting the flow of ions, causing a small build up of ions that, when discharged, cause the popping or cracking sound 3 speed settings. Always turn the unit off and unplug before cleaning. Once unplugged, use a soft, damp cloth and a non abrasive cleaner (optional) to wipe the grill area