First thing is to replace CMOs battery with new one ($1-2).  It could be bios is malfunctioning.  The fact that fan eventually quiets down speaks against bad thermal paste.
After a good number of years with my 755 performing flawlessly, it developed the dreaded Dell "speeding fan issue". The pc was running all night, doing moderately low cpu tasks, when the front fan started revving up like a jet aircraft... waking me out of a sound sleep, of course. That was the start of the issue. Now, the revving fan only does that after the pc is totally manually powered down and restarted... not rebooted in software. When the pc is powered completely down and then started, the fan runs like mad for a hour or two than slowly and mysteriously returns a quiet normal level and stays that way. I saw this in the error messages in the bios: U ... date/time... out of range ... cpu fan. That's the only clue I have. My question:  Is the circuitry that is causing the speedup on the MB or the fan itself? If the sensing circuitry is on the MB, I will replace the MB. If the fan has temp sensing circuitry, I'll replace the fan. Thanks, Ken
Optiplex 755 racing fan issue