Noise coming from underneath the car. It is so loud that attendants at a drive through window found it hard to hear my order. It's been doing it for a while, but the car runs fine.
Hearing any noise coming from your car can be startling, but it can be especially so when that noise is so loud others cannot hear you! Though the information provided is a bit limited, my experience tells me that the noise you may be hearing may be due to a leak in your exhaust system.
As vehicles age their components will naturally wear out and when you add the extreme heat of the engine to the equation, it is only a matter of time until one of those parts completely fails. Now if your vehicle seems to just sound louder in general, then it is likely your car either developed a hole somewhere in the exhaust system or the exhaust gasket itself has failed. When a failure happens, it will allow exhaust gases to prematurely escape the engine bay by bypassing the muffler. This is what you are hearing when you are aware that your vehicle sounds louder.
However, if instead you hear a clanking noise, such as a couple of quarters in a coffee can being occasionally shook around, then it is possible the material inside a component known as the catalytic converter, or even your car’s muffler itself, is failing and breaking down. When this happens the materials inside these components tend to break off in chunks and sit at the bottom of the muffler/catalytic converter and bounce around. This could also be the noise you and others are hearing.
As you can see, without actually hearing the noise it is a bit difficult to narrow down the issue to one diagnosis. Your best bet would be to next contact a certified mobile mechanic to not only use their mechanic’s ear to narrow down what you are hearing, but also inspect your entire vehicle for safety.