Having a hard time trying to imagine what a dun dun sound is while starting.
My first suggestion would be check the air filter, seems like you may have done that.
Next, Pull the distributor cap up and check for moisture/water . that would be second on my list. if you find it damp under the cap, that's your problem, It is more then likely what your issue is if you have no water in the air filter.
5NineZJ,35mtr/k IRO 7" LA F/R Detroit F/HP-4.56 R/Aussie 44A. ARB Snorkel/JRC Sliders/ Protofab w/9500S./Bushwhackers. Novak242 TCcable.