The front wheel bearing seals on 4WD Rangers sometimes make a squeak or kind of howling noise at low speeds. I don't know if 2WD does it. Mine usually does it after going off-road in dusty conditions. It then goes away after a while. I understand you can put a little grease on them to make it stop. I have never lubed them but if I did I would use a silicone spray that will dry out and not collect dirt.
Could the tires be making the noise? Maybe the plies are coming apart and creaking.
Do you have beauty rings or any sort of hub cap? They can make noises.
Get a friend to walk along beside your truck in a parking lot as you drive slow.