Running into the same issue where fans continuously cycling up and down. I have not installed Freenas yet on my build. I am running IPMIUtil on a remote machine running Win7. I changed the lower and upper threshold values to 100/3000 for all 3 case fans only. I left the CPU fan alone. The specs for the fans used in my case are listed here 
https://www.sanyodenki.com/archive/document/corporatedata/technicalreport/2004/TR17e_c_en.pdf
They make no mention of minimum ratings so I can base my thresholds. After I made changes to the thresholds I made sure to reboot and still no changes 
C:\.........\IPMI>ipmiutil sensor -N 192.168.0.180 -U "
Username"-P "
Password"
-c
ipmiutil ver 2.98
isensor: version 2.98
Connecting to node  192.168.0.180
-- BMC version 1.11, IPMI version 2.0
 ID  | SDRType | Type            |SNum| Name             |Status| Reading
0004 | Full    | Temperature     | 01 | CPU Temp         | OK   | 39.00 C
0047 | Full    | Temperature     | 0a | PCH Temp         | OK   | 27.00 C
008a | Full    | Temperature     | 0b | System Temp      | OK   | 24.00 C
00cd | Full    | Temperature     | 0c | Peripheral Temp  | OK   | 31.00 C
0110 | Full    | Temperature     | 10 | VcpuVRM Temp     | OK   | 35.00 C
0153 | Full    | Temperature     | b0 | DIMMA1 Temp      | Absent | 0.00 na
0196 | Full    | Temperature     | b1 | DIMMA2 Temp      | Absent | 0.00 na
01d9 | Full    | Temperature     | b4 | DIMMB1 Temp      | Absent | 0.00 na
021c | Full    | Temperature     | b5 | DIMMB2 Temp      | Absent | 0.00 na
025f | Full    | Fan             | 41 | FAN1             | OK   | 2500.00 RPM<-------------CPU FAN
02a2 | Full    | Fan             | 42 | FAN2             | AboveCrit | 6000.00 R<-------------CaseFAN
PM
02e5 | Full    | Fan             | 43 | FAN3             | AboveCrit | 6100.00 R<-------------CaseFAN
PM
0328 | Full    | Fan             | 44 | FAN4             | Absent | 0.00 na
036b | Full    | Fan             | 45 | FANA             | AboveCrit | 6200.00 R<-------------CaseFAN
PM
03ae | Full    | Voltage         | 30 | 12V              | OK   | 12.13 V
03f1 | Full    | Voltage         | 31 | 5VCC             | OK   | 4.97 V
0434 | Full    | Voltage         | 32 | 3.3VCC           | OK   | 3.37 V
0477 | Full    | Voltage         | 33 | VBAT             | OK   | 2.97 V
04ba | Full    | Voltage         | 34 | VCPU             | OK   | 1.15 V
04fd | Full    | Voltage         | 35 | VDIMMAB          | OK   | 1.21 V
0540 | Full    | Voltage         | 38 | 5VSB             | OK   | 5.05 V
0583 | Full    | Voltage         | 39 | 3.3VSB           | OK   | 3.33 V
05c6 | Full    | Voltage         | 3b | VBMC 1.2V        | OK   | 1.20 V
0609 | Full    | Voltage         | 3c | VPCH 1.0V        | OK   | 0.99 V
064c | Full    | Platform Securi | aa | Chassis Intru    | OK   | 0.00 na
ipmiutil sensor, completed successfully