Hearing A Low Noise/sound From My '07 Chevy

[BAD2500HD]

been hearing this noise of and on since i got the ( aug.'07 ) its a Silverado 1500 ext.cab LT2 with the 5.3L motor its very low like a whinning sound? think once on hear i read about others who said there fuel pumps or ? was what was makeing the sound is this something that is going on with these trucks ? my wife was in it this past sat. and even she wanted to know what the sound was, when i first got the truck i never heard it then maybe 6months it started but just kind of at stop lights or just moveing slow etc. now even going 30mph or more you can hear it and its all the time . what do you guys think and have you heard of anything like this ?