Why do engines emit sounds and where do the sounds come from?

Engines are internal combusion obviously. Most of the time if an engine is making loud noises, it has a problem. A valve might be burnt, a rocker arm might be loose, etc.

Most engines wont have a sound unless theres something disconected or leaking or worn out, as with the exhaust or a vacuum or internal part, some make knocking sounds or a tapping sound meaning there is some excessive space that need taken up, the water pump and the smog pump will also make a noise of loose or worn, what kind of sound are you having and where does it sound like its comming from? If all is good working order there should be a smooth even sound. GOD Bless and good luck.

Moving parts and combustion of fuel. Accessories make noise IE: belts and compressor , power steering pump, starter(When cranking engine). loose parts

Engines turn explosions into motions.

This will teach you about the simplest engine design. The two stroke engine.

http://science.howstuffworks.com/two-stroke1.htm