Okie is correct, backfire through intake is a "Lean" pop. only two ways the 5.9 does it. Either you are lean, or your are firing out of time via bad wires, wire routing, or mechanical timing failure.
First step, put a scantool on the vehicle and check the fuel sync and verify the ECU timing with a timing light and make sure they line up. that eliminates mechanical timing in minutes.
second step, using scantool check your short and long term fuel trims. numbers in the double digit positive mean that you are running lean and adding fuel to compensate. numbers in the double digit negative mean your running rich and pulling fuel. Often times we see issues where the o2 sensors are bad and reading a full rich or full lean condition..... you should be able to check your o2 sensors in the scan tool by watching the numbers move from rich to lean as the fuel system tries to correct itself (in closed loop when warm)
Loud hissing is usually the IAC valve being very open, but the 5.9 has a loud throttle body to begin with. one of the reasons I suggest not using a CAI, and sticking to the stock box, unless you are running forced induction your motor will never outflow the stock airbox, and the stock airbox got it's air from the cold side of the fender to begin with.
finallycheck your wires. inspect them for burns, cuts, traces of carbon lines where they might be leaking voltage to ground or to other wires. Check your spark plugs for wear, check your cap and rotor for carbon fouling and cross talk inside the cap.
If you can't find the issue using the above steps, and you have no check engine light on, I can only assume you have stock heads and you might have sunk a valve seat or or four.
sunk valve seats are hard to diagnose on the magnum as they rarely show up in a cylinder leakdown or a compression test, but tend to make themselves obvious around idle.
some guys use a piece of cardboard over the exhaust to test for it, if the exhaust always pushes the cardboard away from the pipe then valves should be fine.... if the cardboard gets blown away and then suddenly gets sucked up against the exhaust pipe, then usually indicates a sunk exhaust valve. which will cause a lean pop on mild to heavy acceleration because instead of getting the proper A/F mix that cylinder, its drawing hot air back in from the exhaust and causing a lean condition. This is tricky because it usually affects one cylinder at any time and has such a small effect on the fuel trims that it often goes without setting an engine light.