fluttering

my truck seems to have a fluttering sound when i accelerate and press the clutch in. i can only hear it when i rev it up, not when i am pushing the gas, and i hear it all the time when pressing the clutch, until it gets to idle. could this be a case of not enuff air to the engine, or a bad catalytic? what else could this be? i just put all new seals and gaskets, and oil.