Yeah its the load, I am not certain of the specs, but the fans spins all the time just not a full rpm of the motor. When the fan clutch locks it runs the rpm of the motor. Thats why you get the fluctuation in the sound you hear. The worry isn't really that your fan came on. Its the why. Mine only came on when my egr cooler decided to start leaking and then only when I was towing or sitting idling at a light. No air across the radiator. The time the egr cooler went completely out it was on 24/7. Though I have replaced a couple that where just not working correctly. Now I would have to believe that the fan clutch could come on on really hot days when idling and it would go off relatively quickly when the air starts cooling the radiator down. Would also think it would come on when towing in the heat. Though like I said, would be better to hear from mdub or someone with more diesel experience than me. I am still learning this diesel craft.

 

Ive had the truck since December 2011 and notice this sound for the first time right after i had a coolant flush done in June. But we also hit triple digit temps that week. Just going to keep an eye and ear out for anything unusual, cause other than that truck runs great.

 

I don't think my fan has come on fully for a year, untill the other week it was 106 outside and the asphault had to be at least 150 deg. sitting in traffic it came on and supprised me, forgot how it sounded.

 

My fan NEVER EVER EVER comes on, UNLESS.... I'm low on coolant. Check your coolant level. The fan is a loud sumbeech. It does get louder when you get on the skinny pedal, it's driven off the crank through a clutch setup.

I towed my camper with my GoGo race tune this weekend and the fan never came on once. Temps were probably in the 85* range, but it was humid as anything out too. Truck is a turd in the high humidity.