Ever since I got my truck two years ago, there's been a hissing/grinding noise that I could hear only when the windows were open. It actually sounds like a cat hissing or the "H" sound at the beginning of "he." I hear it primarily through the passenger's side window. At first, I thought it was the front bearings, but they are all new, along with front axle U-joints, rotors, calipers, pads, etc. It is actually louder with the new bearings and has gotten noiser over the past few months. It is quite noticeable as I drive. Nothing feels amiss, however.
My thoughts in order of likelihood:

1. Bad hub bearings (again?). They're only a year old. I don't much feel like going in after them again. There was a similar sound with the old bearings, which were fine and only replaced because they were destroyed when I tried to remove the hub for the new rotors.

2. Something in the driveshaft (I know it needs U-joints and maybe a carrier bearing).

3. Front tires, which are old and have developed cracks on the inside (no, I'm not driving on them until I get this resolved).

Like I said, it's somewhere between a hiss and a grind. Any ideas that I haven't thought of? Thanks in advance.

have you tried to put all 4 wheels up in the air with the truck on some sturdy wood blocks?

With the truck in the air, you could have someone get in the driver seat, start the engine, and rev it up a little bit in order to repleat the sound when the truck is in drive and your outside listening for sounds like the one you described.

By the way, have you checked your bushings and front end components for excessive wear? It's possible that if a bushing or front end component is serverly worn somewhere, you'll get some noise from the constant movement associated with the joint. Check it out, but be safe. Make sure that if you start the truck and put it in drive while up in the air, that nothing is going to move, or go anywhere. The last thing you want is to get run over or hit by your own truck when you are under or around the truck.

i had a dodge that made some noise like that and they found out it was the four wheel drive and the front drive in the punkin.

Check out the sheild that goes around the brakes.. if it rubs make that noise... I bent one of mine when i did the bearings. Sounded like what ur discribing. Just a thought