Hi:
I have been pulling a fifthwheel for about 2 years (I have F30 2005). And this is the first time I experienced the same sound. I was quite worried since we are in the middle of nowhere, far away from home (In NM but live in Alaska). The dealer assured me it was okay and gave me similar answer: Not to worry but watch it!
So I am surmising that these could be the reasons:
1. Fan kicks in as stated in your post.
2. Sometimes tow/haul will kick in as we deccelarate at the top off the hill. My "noise" has been happening when I would take a foot off the gas pedal while in tow haul mode.
Would anybody would have any comments on this phenomena? Thank you Kid