I list this topic first because it's probably the most common. Some engine noises are normal; others can mean your engine is about to come apart. It's important to decide what category your noise fits into.
The engine noise that’s the most concerning is a knocking noise. Knocking noises can mean loose parts inside the engine. Loose parts inside the engine can cause catastrophic results. The knocking noise I'm referring to here has a slow frequency; it's not a fast noise, but more of a rhythmic thump at idle, which increases with engine speed. This noise might also coincide with a blinking oil light on the dash.
If this is the case, you likely have loose engine parts that need to be serviced or replaced. Many people see this as an oil pump problem, but this is incorrect in most cases. Oil pressure is actually created by the clearances between engine parts. If that clearance becomes excessive, you will see a drop in oil pressure. Oil pumps create a volume of fluid, not pressure. It's the resistance to that flow that creates the pressure. With your engine, that resistance is created by the clearances between the moving parts. Here's a video that will help explain what I'm talking about.
A knocking noise from your engine can also mean that you're low on oil. One of the first things to do if you hear a noise from your engine is to check the oil level and condition. If the oil is low, top it off and recheck for your noise. If it goes away, consider yourself lucky, but be on notice. If your oil was low, there is a reason. It could mean you have a leak, or it could mean that your engine is burning the oil in the combustion chamber. Either way, it's a situation that should be monitored closely. An engine without oil, especially one that's making noise, is not long for this world. I usually keep a spare quart or two of oil in my vehicles to deal with this very scenario; I would suggest you do the same.