You may want to apply latest BIOS/BMC/FRUSDR if you haven't done so. Current version is BIOS 98, BMC 65 and FRUSDR 48. You can check the versions in BIOS: Server Management => System Information.
On SR1550, fans #1-4 cool down the CPU/Memory/Low Profile PCI zone, while fan #5 cools down the full height riser zone. Check if all air ducts are properly installed in your chassis. If you're still seeing same issue after applying latest BIOS/Firmware, check if your PCI device on the full height riser is generating too much heat.
Thanks for your help
For some reason the Intel Deployment Assistant CD did not download the newest updates from the Internet.
The problem was solved by installing new firmware versions manually:
Now running: BMC 0.65
After the updates, the FANs are now running normally.