Okay, I have a 2018 F150 XLT, Crew Cab. Last Sunday I'm driving to the ski hill. Temp ranges 2 below to 12 below zero. Heavy fog in some areas. Roads are clear and dry. Set cruise control 70. About 3 miles down the road I begin hearing this "steady" medium high pitched tone. I turn off the sat radio, still there. I turn off the heated/defrost fan still there. Due to the fog getting pretty heavy in some areas I slow down and tone quits at 60. I speed up and it comes back on.....slow down it goes off. Fog thins, but is still there, lets me speed up to about 64 and starts in again. Wife is scouring the owners manual. Only thing there is, is about collision avoidance and a dash warning light and tone. I was not getting a dash warning light only a tone. Nothing in the manual about fog and limited visibility setting anything off. Cannot find any postings or articles about this. Once out of this area and fog was gone, no more steady tone at whatever speed. I don't keep up with all the new technology that's out there. I don't have a following to close/collision warning on this truck to my knowledge. Anyone heard of this? Thanks!