my fourwheelers do that all the time, all i could tell you is that it happens when the engine is cold and won't take the fuel as fast as it's going in or it's running leaner than it should be. it's like backfiring except through the carb rather than the exhaust. as i said, this is on my fourwheelers, not a truck, so i'm not sure if it would be the same reasoning.

or as the guy above me mentioned, you could have the A/C or heat on, the truck will idle up and down whenever the pump kicks on to run the heat or A/C

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1996 Ford Ranger XLT 245 4.0L OHV V6 4X4 Supercab-No Cats-Chipped-Flowmaster Original 40 Series SI/DO 2.25" Piping/3.5" SS Tips-15x10 Directional Rims W/ 31/10.50R15 Cooper Discoverer ATR Tires-Summit Racing 3" Body Lift-Rough Country 4" Suspension Lift-Rough Country Nitro2.0 Shocks-Memphis 1000W Amp/Two 15" Hifonix Olympus Competition Subs/Vented Box