1995 Chevy Corsica Knocking

Ok, my car makes a knocking noise when at operating temperature that you can only hear from inside the car. You can't hear it at all from outside the car. And the noise is only at idle. Oil level is full and there is no oil pressure light on. I am using synthetic 5w-30 in my car, could this be why? should I switch to conventional and would that help?

thanks