Motor Noise

hi, I HAVE AN 1986 T/A. it has a 350 TPI motor.(not stock) every once in awile i will get a knocking noise from the motor,,you can't hear it unless you open the hood and really listen...if i put my hand on top of the tune port i can feel vibrations that courespond with the noise..i crawled underneath it and you can't hear it at all...the car runs great ...tears the tires off...it doesn't make the noise all the time...sometimes i'ff drive it all day for a week..and won't hear a thing ...then i'll drive it around the block and hear it again...any recomendations????...HELP ME