ex4200 fan speed spin at high speed with normal condition

Hello,

I just installed many ex4200 as top of rack switches in my rack but their fans are spinning at highspeed, 

i check temp and it was ok, 

i touch the switches they were not hot and everything was ok.

i execute "show system alarms" "show chassis alarms" there were no alarms,

also there are no servers or devices installed in rack so the only devices in my rack are switches,

so is it normal?

thank you.