[NAME]  If your PC is under warranty, contact Dell Tech Support. Also confirm your power button setting is to turn PC off, not put it to sleep.  Go to Control Panel> All  Control Panel Items > Power Options > System Settings and change the power button settings there, if necessary. If that doesn't help, make sure all your drivers are up-to-date. especially Intel Management Engine. What version of BIOS is running?    
I am used to dell fans running loud on boot up...had this on my old XPS Studio. With my current XPS 8940, all updates applied, etc., there is no fan noise when working, but putting the unit into sleep mode, boy, the fan noise is full tilt. I can take the unit back to Costco, but would rather get a fix for this issue.
XPS 8940 fans quiet when working, full speed when sleeping