For a few weeks, the fans of my Lenovo B590 laptop, running on Xubuntu 14, turn to high speed a few minutes after it is turned on. The fans won't speed down until I turn the computer off.

This is quite strange, since

This didn't happen before

The temperatures are quite low (are they ?)

$sensors
Adapter: Virtual device
temp1:        +36.0°C  (crit = +88.0°C)
temp2:        +30.0°C  (crit = +126.0°C)

coretemp-isa-0000
Adapter: ISA adapter
Physical id 0:  +37.0°C  (high = +72.0°C, crit = +90.0°C)
Core 0:         +34.0°C  (high = +72.0°C, crit = +90.0°C)
Core 1:         +31.0°C  (high = +72.0°C, crit = +90.0°C)

thinkpad-isa-0000
Adapter: ISA adapter
fan1:           0 RPM

pkg-temp-0-virtual-0
Adapter: Virtual device
temp1:        +37.0°C 

$sudo hddtemp /dev/sda
/dev/sda: ST500LT012-9WS142: 33°C

The computer is under low load:

top - 08:30:15 up 16 min,  2 users,  load average: 0.28, 0.23, 0.23
Tasks: 197 total,   1 running, 196 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.8 us,  0.5 sy,  0.0 ni, 98.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:   3607944 total,  1973956 used,  1633988 free,    99660 buffers
KiB Swap:  3744764 total,        0 used,  3744764 free.   789936 cached Mem

The BIOS is up to date (and there are no fan settings in it)

The fan is clean and dust-free

Why would the BIOS turn the fans to high speed where there seem to be no reason for that ?

It seems that we cannot control the fan manually with this model, so I guess the only solution is to understand why this happens.