Jer82Z28

On my 87 Iroc the engine has started to knock while idleing, regardless of it's operating temperature, the engine runs rough and the knock seems to be timed to its shimey, as if the engine itself is hitting against something. the sound is very similar to exhaust tapping against the floorpan, however i cannot find any external reason for the knock. As soon as i bring the engine above idle the knock disapears, as i rev it up i get a different vibration and sound, almost like lifter rattle but only between 2200-2500 rpm, and only at a certain amount of engine load. I can feel this vibration through the gas peddle. It's an elusive vibration thats hard to recreate. The engine otherwise runs well, has plenty of power but smokes oil when warm. It uses about 1L of oil per oil change.

Is it possible this knock is the internals of the engine? If it is internals it sounds like it will grenade any second. Its a horrible sound yet i have put on over 3000km since it started.
any thoughts???