Metallic high pitch friction noise
 

Friends.- My '06 RR. 4.4 v8 does a High Pitch metallic friction noise when is warm and has run for several miles.. The noise originates near the front of the vehicle, under the radiator protector. It could come from the hidrolic steering pump (but doesn't stop or increase when the steering wheel is moved.... I could come from the A/C compressor but doesn't stop when the A/C is shut off... It could come from the alternator.... or from the tensioner pulley or even the drive belt.... ANY HINTS? The noise stops suddenly and doesn't come back for a while. The first time I heard it was when I was climbing a steep and full of curves' hill. My truck has 50.000 miles. ANY HINTS?