If your car is making a rattling sound along with ticks or slaps, you should see if it's coming from the front of the engine. It might help to buy a cheap stethoscope to aid the listening process. It can really help! Listen also for the sound to rise and fall with the engine RPMs.
If the rattling ticking slapping sound is coming from the very front of your engine (or the side that has the belts on it) and gets louder or faster when you rev the engine, chances are you have a loose timing chain. Not all cars have a timing chain, but lots of them do. If you catch a loose timing chain early, you can fix it before things get ugly. A broken timing chain can lead to serious engine damage, so if you suspect your timing chain is going bad, don't hesitate to get it looked at.
If you're hearing just the ticks with no rattles or slaps, it could be your lifters hollering at you, so check that out, too.