Howling sound from my engine.

There is a bad bearing, probably in a serpentine belt idler wheel.

Take the belt of and briefly start the engine. If the noise is gone then it is something the belt drives.

You didn't say if you have a standard mechanical fan or an electric one in teh truck now, but it sounds as if its probably a mechanical one. IF SO, it sounds as if the fan clutch isn't operating properly-and if that's the case it would fit fairly well with what you're describing. When the engine first start (from cold, not a restart) a normal fan clutch may make the fan spin for a VERY short time as the viscous fluid inside it takes a bit of time to thin out-that's ok, as long as it releases quickly, otherwise there will be a lot ofunnecessary fan noise (especially noticible at at low rpm) and while the engine is still cold. The fan blade itself id SUPPOSED to be freewheeling whenever the level of heat that gets to it (the clutch) is BELOW a preset temperature. If it does NOT release COMPLETELY when the temperature is below that set temperature, the dragging (somethime rust-LOCKED) fan blade will continue spinning when it doesn't HAVE to-which will make a lot of "extra" fan noise at a lower rpom rang- where you can hear it more easily. . Now, when the engine RPM is high there is usually a lot of airflow past the fan clutch, which, in YOUR case may be cooling the clutch enough for it to mostly release. Either that or the fan blade is flexible enough that it flattens out significantly at higher rpm-which all by itself would reduce fan noise, even if teh fan clutch is not releasing at all. Easiest way to tell if its a bad fan clutch is to try to spin the fan by hand when the engine is cold. It may be a little hard to turn at first, but if its a fluid filled one it SHOULD quickly get easier to turn. If it does NOT quickly loosen up its bad. To double check tat's the case, , stop the engine when its HOT and the fan blade should be very HARD to turn- If its ISN'T hard to turn with a hot engine, its bad.