Depends on the engine. Best case its just a loose tappet or a worn hydraulic
lifter. The 1.5L 12v 4G15 engine has adjustable roller tappets. If this gets
loose, the noise is a metallic ticking sound from the top of the engine.

The 1.8L 16v 4G93 has I think hydraulic lifters. These give up when old and
get noisy. One way to check is hold the revs to 3000 for about 30 seconds,
then SLOWLY let the revs drop back to idle. If the knocking sound goes away,
its a lifter.

Worst case its a big end bearing. Starting at idle, blip the throttle. A
worn bearing will sound like a hammer hitting an anvil, and comes from the
bottom of the engine. Often theres a no load rev point (about 1800 rpm
usually) where the knocking stops. Blip and it knocks, reduce revs and it
starts again.

If it is big end, it needs to be addressed immediately, as severe crank
journal wear occurs very quickly. The "good" news is that its possible to
drop the sump and look at the big ends. If a bearing is shot, its simple to
replace if theres no crank damage. If there damage, the best option is to
get another engine from a wrecker and rebuild it, then swap it into your
car. Or throw the car out if its a rust bucket.