The main problem is that dell uses 5 pin fan headers so even if you get a Noctua 4 pin you won't get PWM function. The CPU fan seems to do a good enough job so I don't really even care about that front fan working. You might have to press F1 at boot up to avoid the warning that the fan is disconnected. Haven't seen how to disable that in BIOS.
In the USFF the Copper heatsink (+3 copper pipes) on top of the CPU is nearly good enough (with good thermal paste) that you don't really even need the CPU fan plugged in. Then it's completely silent. You should only do this though for general use like reading email or internet surfing which is not CPU intensive.
View attachment 386369