It sounds like the normal cold weather valve tapping of the second gens. If it is whining though, it will be the timing chain guide. The whine will grow with The RPMs.
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2007 Xterra S 4x4
Wheels, Tires, and Suspension
285-75 R16 BFG AT-KO, Pro-4X Wheels, 2" OME Lift
NXRocks Skids, TAG Rear Bumper, White-Knuckle Off-Road Rock Sliders
Xoskel Bumper Mount Bracket, Blacked out Emblems, Weathertech Vent Visors, FOURTREKS Hi-Lift Mount, Wilson1000 CB Antenna
Pioneer AVHP3200DVD, Midland CB, Compass Mirror, Nissan All-Weather Floor Mats, Nissan Carpeted Cargo Mat