It's your transmission down shifting. The noise you hear outside is more likely the aux cooler cooling the transmission. What happens is the fan speed will increase noticeably to help get air to the transmission cooler during these downshifts.
Broome101
2005 F350 PSD,SRW,CC,LB, King Ranch, 4x4
2005 3685FL
http://www.picturetrail.com/broome101