If they are worn out they would be missing under a load...were they? All the same is good. Did you put the exact same plug back in there?
If it is an exhaust manifold you are asking about it should make some little noise when cold and idling.
If you are asking about the intake it should miss when at idle.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist