1999 Silverado engine rocking

1999 Silverado 4.3L....When it has been sitting over night first of all when turning the key just until the radio turns on the radio will flicker on and off.......then when turning key to start the engine will start but engine seems to rock at a very slow idle. Occasionally it will turn off needing to be restarted......After the initial start and it warms up the rocking stops and there is no trouble while driving or while starting.....shop said I have piston slap but from what i've read it sounds more like a hammer than just a rocked/vibrating engine......thanks for any advice.....also it had a p-303 code...Checked oil color and level seems normal....shop said it needs new engine or full rebuild.....suggestions appreciated