My engine (Ford Escape Hybrid 2006) has gotten noticeably louder over the years. Not to the point that I'm concerned, but it has gotten me wondering why exactly engine noise gets worse with time. I've heard about piston slap, but if that's not occurring then what else? Do the belts get louder? Bearings? I am just curious.
I take it that you're referring to the noise of the engine in the engine bay rather than the exhaust.
I'm personally still amazed by the number of parts that go together to form an engine (and a car in general). Over time of course, almost all of these parts will wear. Serpentine belts will wear. Non OEM air intakes can be very loud (modifications are often not wife-acceptable). Timing belts will wear and extend, putting camshaft timing out. Timing chains will also do the same, creating more noise especially if the tensioner is not working properly.
In my experience, most of the noise is usually generated by the valvetrain. It tends to go out of adjustment over time and will be the noisiest when it is cold - which is usually when you decide to inspect the engine. I would check to see whether your vehicle requires a valve adjustment if it seems excessively noisy. I rebuilt an inline six cylinder Ford engine once and the valvetrain went completely off 2 weeks later. The noise was incredibly loud when driving it.