Knocking sound

The engine makes an obvious tapping/knocking like sound. It's hard to tell where it's coming from. The top engine has noisy valve train but can be heard well throughout engine. Easier to hear at bottom engine but unsure if that's the location since the sound travels. The sound is loudest when idling or car is at very low rpm's. Doesn't matter what gear. Once the car starts moving, the noise isn't really audible but gets loud as soon as car stops. It does burn oil a little sometimes but I have been making sure it doesn't get too low. Engine light is on for catalytic converter and EVAP leaks. Also loss of power and transmission shifts noticeably. Not sure if it's all related. But the engine noise concerns me. It is a constant metallic ticking. when revving the engine, the noise will become drowned out and quieter but the tapping gets faster. Sometimes won't be heard too loud right at start up if cold. Oil pressure light will always come on for about twenty seconds when car is first on but stays off after. Could it be upper engine noise such as lifters or rocker arms or a dreadful rod knock?