I have this Dell 2900 running ESXi 6.5 near where I sleep and I wanted to turn the volume down. This server screams! First I tried pulling some of the fans out and I found out that it make it scream louder. I cut my fan speed in half by soldering in a 22 ohm 5% 2 watt resistor on the red wire of the fan lead. I first tried a 47 which slowed it down pretty good but tripped the alert on the machine to kick in all the other ones to make up for it. 22ohm is the way to go.
I only did the fan mod on fan 1, fan 4, fan,5 and fan6. I wanted to leave the two over the processor. I might change this later, but for now it sure it more quiet.
Here is a photo of the resistor on fan4, the one over the ram. This one seemed to be the louder than the rest. I followed the red wire from the fan to this connector figure there was more wire for me to mess up on. All I had was trash solder that came with some trash soldering iron.