You may have some foreign object that is intermittently interfering with the fan caught in the fresh air intake for the cabin air. This could be anything from a defective cabin filter, to an oak tree leaf that has been worn into submission by daily use/abuse/neglect. It could also be a mixing door that is not closing properly when a particular setting is requested by the climate control system. Does this occur when you have the system set to anything in particular? Or, does this happen specifically in the DEF mode or the vent mode or
floor mode etc? Sometimes cold weather can cause the lubricants in these systems to become less viscous and wreak havoc with noisy hvac components. Try operating you climate control in different modes to see if the noise still persists. (mine has been working fine in temps as low as -30F!)