2003 Ford F-150 ticking noise

I have noticed for the last two days when the truck is driven forward under 10 miles per hour, it makes a ticking noise under the undercarriage around the area of the transmission. The faster in speed I go, the quicker the ticking noise gets. The ticking noise does not get any lauder, just faster. However, after the speed exceeds 10 miles per hour, the ticking noise goes away. I recently came back from a trip where I towed my 23 foot toy hauler.