Report "Lasko E20739 makes a knocking noise. How do I fix this? I disassemble the fan once a year to clean thoroughly. The knocking does not last all the time and if I toggle between speeds, the knocking stops for a few moments. My only guess at the issue is that the blade's spindle must be loose and not 100% vertical causing high speed bumps against the housing walls which sound like knocking. This problem happens more frequently as the fan ages"