Usual Suspect wrote:
> why do car engines get noisier with age?

my $0.02... until they're broken, they don't get that much noisier.

However:
the exhaust system starts to rust out and spring pinhole leaks...
aftermarket muffler put on changes the tone...
the underhood insulation falls off (like my Jimmy) and the engine
appears to get louder.

For a typical car, it's a gradual thing where the parts that are "wore"
but not broken all add up to a noisy engine:

fuel injectors,
a bearing in one of the pulleys,
the power steering pump,
a pinhole exhaust leak,
the underhood insulation ripped out,

and so on... combined together it makes for a "loud" sound instead of a
"nice" mechanical sound.