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
Oil color and level seems normal, I have noticed a lower mpg recently suggestions appreciated

 

Engine Rocking Please help if knowledgable on the topic

 

If you mean running rough by engine is rocking, and mileage is a little worse than usual, you might want to check your intake manifold gaskets. It run rough ubtil it gets warm then it runs fine. If you let it go to long you wil start throwing codes P0171 0172 0173 0174 these are banks 1 & 2 / oxy sensors lean running conditions. The truck will start idling from 2000rpm - 200rpm cold warmed up it idle ok but it will start idling low 350 - 400 rpm.
The gaskets, good ones Felpro high performance (Steel Gaskets) cost about $75 at autozone or advance. About 3 hours with hand tools and you're good to go.

 

Could that be what it is wince it is already throwing a p0303 code? How can I test that theory without actually taking everything apart?