I had the same problem with a car I used to have, it turned out it had a bad condenser, only after it backfired so much it blew apart 2 mufflers. Try giving it a tune up and see if it doesn't solve your problem.
Your question is a bit confusing. If you are really hearing a backfire noise from the intake then the only thing it could be is your valves are out of adjustment or out of timing. It would be noise of the cumbustion in the cylinder chamber being allowed to exit through the an intake valve.
It could be a bunch of different things causing it one thing could be that it is loading up with gas and it is backfiring or it could be that you need a tune up done on it you would be surprised the problems having bad spark plugs will cause in the vehicle.
Most likely your truck is out of time. It would seem either the distibutor has slipped or the mechanical timing is off. Set the crank and cam shaft at TDC and check location of rotor.
