Can you check whether you have a Standard or High Performance Fan? R740xd with rear backplane need high performance FAN. You can refer below link for FAN requirement for various configuration [URL] 
Hi all, The rear backplane on my 14th gen R740XD (iDRAC9) is causing the fans to run at 100% constantly. Does not matter if it's occupied with disks or not.As soon as I disconnect the rear backplane and start the server again, the fans run at normal speed. I've been searching online, but can't seem to find users with a similar issue. Any ideas? Any additional info I should provide? BIOS version: 2.14.2iDRAC firmware version: 6.00.00.00 Thanks in advance!PJ  
R740XD fan speed at 100% when connecting the 4x 2,5" backplane