running bad

[cacauba]

okay first off my motor is the original 350 and has 160k. needless to say it needs rings and has no power. but recently i have started driving it again daily, and have noticed a very strong ratteling sound from the motor but only under load. i check for exhaust leaks but found none, then i replaced the splugs and wires. the ratteling sounds like a missfire or and exhaust leak but very rappid. i know this is nothing to go on but i cant figure it out, and if it's something simple then i will just feel stupid not fixing it. my next thoughts are that it is the rocker arms, but i'm not sure how to check them for looseness. let me prethank anyone for there input