Here's the firmware image that I just used to downgrade to 1.70 to solve the fan noise issue in my C2100:
https://www.dell.com/support/home/ae/en/aebsdt1/drivers/driversdetails?driverid=hh9ww
I downloaded PEC2100Bmc170.zip and then uploaded the enclosed firm.bin file via the BMC web interface. Note - I had to choose "Forced" for upgrade type instead of Normal because otherwise it kept throwing the same checksum error as mentioned above (0x84). Assuming this was just because the firmware version was less than what was currently installed.
After updating for about 10 minutes, system rebooted and the fans gradually stepped down from 5,000+ rpm to 3,000 rpm and it's finally tolerable to be in the same room with it!
