Throttle Position Sensor.

 

my truck does the same "roar" but it is only first thing in the morning when i get into it. it only lasts for first gear then its fine for the rest of the day.

 

The roar may be the fan clutch on its way out. It is not supposed to engage at cold temps. You could check this by popping you hood and trying to turn the fan. If it doesn't turn very freely, then the clutch is partially engaged. This could affect your mileage.

 

The fan will make that roar noise usually when it's running hot, with the a/c on

 

As it should. Heat causes the clutch to engage. On a cold motor, you should be able to spin the fan with relative ease (a few rotations if spun by hand). If you can't, you mostlikely have a failed or failing clutch (i.e. partially or fully engaged all of the time).