There are rubber hoses running from the air pump to hard lines on either side of the motor, near the valve covers. Where these hoses connect to the hard lines are one way valves. The drivers side line connects to a teed hard line at the rear of the motor. This line connects to the rear of both exhaust manifolds. The passenger side connects to the passenger side exhaust manifold near the header pipe. When the hoses or lines break, it sounds like an exhaust manifold leak, only deeper and less metallic sounding. Check around the air pump and those valves as well.
I'm only guessing here. You didn't mention how the truck was running after the sound started, so I'm assuming there was no change in performance. Mine did pretty much the same thing.