Possible Cause          


Recommended Action

General Fan Noise The fan could be running in stall.  

Check the actual fan performance against the fan curve to determine where on the curve it is operating.  If it is to the left of the peak pressure point the fan is generally in stall.  If in stall see if the ductwork can be modified to eliminate high pressure loss sections.
Low amps could indicate the fan is in stall.  Reduce the pitch angle.

Bearing noise varying from a 'dry' rumble to a squeal Improper greasing   Check the bearings are packed with the correct grade and amount of grease.  If the bearings are of the sealed-for-life type replace them.
  The bearings may be loose on the shaft or bearing support   Tighten the adaptor sleeve or collar.
  The bearings have not been properly tensioned   Re-tension as required
Rough lumpy sound The bearings may have brinelled.  Brinelling occurs when the fan is vibrated during transit or through ground vibrations when stored.  This causes the bearing to vibrate at a single point and therefore indenting the bearing race.  Problems with brinelling usually occur shortly after a fan is installed.   Replace the bearings.  To Avoid brinelling the fan impeller should be rotated frequently, at least daily.
  Excessive belt tension   Check and adjust the belt tension
Shaft seal squeal The seals may have dried out.   The seals may require lubrication or may be misaligned.
Air noise Air noise can be generated from a number of sources such as grilles, bends, badly designed duct fittings, excessive duct velocities etc.   Check the duct design is such it does not generate turbulence and therefore noise.  An attenuator may be needed to resolve the problem.
Vibration The impeller may be out of balance.   Site balancing may be practical but, if not, remove the impeller and balance.
  The shaft may be bent.   The shaft will have to be replaced.
  Impeller may be worn as a result of handling abrasive or corrosive materials.   The impeller will have to be replaced
  Material such as dust or grease could be sticking to the fan blades.   Clean the impeller blades.  This should be done on a regular basis if dust and/or grease are constantly present in the air being handled.
  The impeller may have been damaged by loose material in the duct system.   Depending on the level of damage the impeller may have to be replaced.
  Noise being transmitted through the building structure.  

Check vibration isolators have been fitted and, if fitted, check they are correctly positioned.  Vibration isolators are fitted but not strong enough.  If bottoming, adjust if possible or replace with more suitable mounts.  Fit flexible connectors between the fan and ductwork.