i recently developed this humming noise when the truck is just sitting there at idle... sounds just like a power steering pump that is low on fluid.. but the power steering fluid level is perfect and works just fine.. i replaced the serpentine belt and all noises went away for about 3 days but now there back!.. sometimes when i put it in gear it will quiet down and even go away but comes right back.. any ideas?? i checked the tensioner and there is no slop and still has alot of tension on the belt.