A few years back, I (unknowingly) had a computer that had the BIOS quiet feature turned on, & the end result was quite the opposite of quiet, it would go by itself and for no apparent reason from quiet to Space shuttle liftoff, then back to quiet.
If you are not used to going inside your Bios, there are lots of nooks and crevices in there, but I bet if you look everywhere, you will find the speed/quiet feature. A telltale sign you're closing in is when you see rpm numbers (revolution per minute of your fan). If you want to find where's the dang thing a little faster, go to Dell's support website, you should find your specific* bios pdf manual
*During boot, it should display the BIOS revision # you're running