Yes it does take a fair amount of work to quiet the Dell C6100 Servers but has been done by many. Part of the challenge is the stock fans used by Dell are 13,500RPM PWM fans @ 72dba each and dell uses non-standard wiring.
- L55xx L56xx Series Xeon CPUs use less power and make less heat you need to cool. Important when using slower quiet Fans (very inexpensive to buy Xeon L56xx CPUs on ebay)
- Substitute San Ace or Supermicro fans, rewire non-standard fan headers to plug into the C6100
- User RimBlock in that thread may still be selling drop in fan replacements
This website forum discusses using these servers for homelabs, here is one of many detailed threads on Taming the C6100 to be quiet.
If you limit to one horizontal row of 1 or 2 nodes, you can also use much larger CPU heatsinks ~2U which makes it much easier to cool. Currently they use smaller than 1U heatsinks.