It could be a number of things, but start with a run of the Dell diagnostics, which will test the operation of the fans. You may also want to examine the fans to make sure they're OEM Dell and that they have not been replaced with non-OEM fans at some point -- there should be a Dell part number on the fan assembly.  
Recently acquired this XFR 6420. It ran hot compared to what I'm used to. Bios diagnostics results log showed 3 "thermal events" and 1 "fan stopped" all dated 3 months before I got it. I replaced the pads (1.5mm) and CPU paste which helped quite a bit to keep it from reaching high temp as quickly. The fan test in BIOS shows "target 2500/actual 3500" and "target 4900/actual 3900" all within specs of 40 percent variance. But this one seems to only start when the CPU reaches 92c and totally stops about 70c. Never throttles or runs more or less than about 3500rpms. (guessing by sound) Is that normal for this model, or could there be an issue in the motherboard or possibly the fan signal-to-board? Bios updated A24 and everything seems to run great, just concerned about the longevity. Thanks.
6420XFR