Unfortunately there is no easy supported fix but if you are brave and love to tinker, your problem may be resolved but your custom modified server is then unsupported by Dell.
If you are interested, look at the web page Dell Poweredge 2800 | tufteln. Within this page, you should see "main how to posts" on replacing existing fans with quiet fans on a PE2800 and how to subsequently modify the BMC firmware to adjust the fan threshold and avoid annoying speed hunting of your new quiet fans. Also this site may be helpfull. Information from these sites may be applicable or adaptable to your PE1750 
Failing a desire or ability to tinker, consider buying ear muffs as Dell will not expose these thresholds to the owner via a BIOS change.
Obviously Dell doesn't want owners changing BIOS settings with resultant overheating of components causing warranty claims (which i can fully understand) but these servers are long past warranty and giving owners the option to re-purpose these old machines and turn them into quiet(ish) home server (or just use them to learn about Dell servers without going deaf) would be nice gesture on Dells part 