Shakey idle
 

For some reason my tuck has a weird idle. It sounds and feels strange. Has a shake to it . Thats the best way to explain.It runs great and only does this at idle. Does not matter if truck is warm or cold . But will not do all the time...Ill feel it at one light then it will be ok then it will do again at another stop. If i give a little throttle it seems better. Any ideas? fuel filter maybe?