The signal may be a warning signal, called "fan failuresignal".

Fan failure signal

If a reduced fan speed (RPM) is detected on any of the redundantpower supplies, it gives out a predictive failure signal. Power offthe chassis and remove the redundant power supply that produces thesignal. Clean the unit if necessary. Put it back in and see ifsignal reproduces. If so, check the warranty of your product withD-Link Europe Ltd.