Despite of any hardware, loading and ambient temperature change, the power supply fans speed up (found out from the increased sound level). However panel diagnostic LED's and Modular Server Control show no signs of anormaly. Fans now spins at 5500 - 6100 rpm and inlet temp at 35-36 degree Celcius. All are still indicated to be within the green zone in the control console.
Any suggestion and insight regarding the cause are welcome. Thanks.