As cars grow older, they often develop many rattles; however, if the rattle is coming from the front of the engine along with ticks and slaps, it sometimes indicates a loose timing chain. If the chain breaks, it causes serious damage to the engine, according to About.com.
A cheap stethoscope is helpful in narrowing the location of the rattle. The device helps to amplify sounds nearby while blocking some of the background noise. Having a partner in the vehicle who is able to press the accelerator pedal to increase the engine's speed allows the individual checking the engine to see if the noise is louder or faster as its speed increases. These symptoms increase the likelihood that the problem is a timing chain.
If the rattle is not coming from the engine compartment but seems to be under the driver's seat, it is most likely a loose heat shield from the exhaust. About.com describes this sound as rocks in a tin can. If the car has been in operation within the past hour, the exhaust system is hot enough to cause burns, so the best practice is to allow time for it to cool before proceeding with an inspection. The shield is located in a position under the driver's seat. Usually tightening a single bolt is all that is required to end the rattle.