Hello The system will have varying levels of acoustic output based on utilization. If the system is at idle it will be very quiet, but if the system is being highly utilized it will put out much more heat and cause the fans to run at much higher RPM. If this is a parity array then a rebuild will cause the controller to perform many calculations above normal operations. It will also cause all of the drives in the array to put off more heat while the controller reads from each of them. If the fans continue at high RPM after the rebuild is complete then I would suggest reviewing the thermals throughout the system. Thanks
Hi There, I've posted this thread earlier about a disk failure in my server. And now I'm waiting for the replacement disk by Dell. However, I haven't shutdown or restarted the server till there was a power cut yesterday. In the meantime, server never had such a noise earlier as it was running smoothly. After the power cut yesterday, I have shutdown the server to make sure it get a proper power off, then switched on the server later when the power was up. I can make sure I didn't do any changes except for the disk failure. Once the server was boot up, it started to produce the loudest noise it could ever do. So, I have done another restart just to make sure that I don't miss anything. However, it started the same noise but it was average not the loudest as before. So, I'm assuming this is because of the disk failure and the hardware or the RAID is trying to rebuild/find the failed disk... I'm not sure just assuming.. Any advise will be great. 
PowerEdge R720xd outputs an average noise