mechanic told me it was "harmonic" whine comming from the waterpump pully and extending to the fan/fan clutch. This occurs when temps hit below your rated cooling fluid. (like a 50/50 mix should be more of a 60/40 in colder climates.) Also, the clutch oil hasn't warmed up on the fan clutch & the waterpump pully is fighting the cold fluid, plus the fan is struggling in the colder air....He said if this is a constant problem, install a heater for the block & plug it in when it's gonna be cold...Not really an option here in Az....lol But I did notice since the other day, I started w/max heat & blower motor on & have not had it since...only 3 other days where it was as cold as that day...Urban legend or physics, it seems to have worked...(knock on wood)