Just wanted to follow up on the resolution to this issue. The firmware and hardware configuration were fine, the problem ended up being a defective motherboard. Dell sent a tech to replace the motherboard, and once that was done, the fans were running normally. The tech said that he's seen this several times, and replacing the motherboard is the fix. Hopefully this will help someone else. Thanks to Chris and the other folks at Dell for working through this. 
My R720xd is running all of the system board fans at full speed continuously. On powerup, the fans bump to high speed for a second, then settle down to reasonable speed, and stay at reasonable speed during POST, right up to the point where iDRAC initializes, at which point they all kick up to full speed, and stay there. From that point on, the fans go full speed. Nothing short of a reboot or shutdown will stop them. I'm not exaggerating even slightly. The fans run at 17,000 RPM, 24 hours a day. It's so loud it can be heard through the walls on the other side of the building. There is no OS installed on this machine. I've upgraded to the latest firmware, tried changing the performance profile in the bios, and tried the whole hold down the power button while unplugged thing as well. There's no possibility that it's overheating. Inlet air temp is 22 degrees, exhaust is 27, CPU temps are 35-40 degrees. I've found other references to people having similar problems with R720s, but there doesn't seem to be any single common cause for it. Any suggestions?
R720xd Fans insanely loud, running full blast