"If you worked on the exhaust and the car started producing a rattle it did not have before you laid hands on it, wouldn't you investigate and fix it rather than handing it to the customer that way? "

Me? Absolutely.
However, if it wasn't rattling at the time I released the vehicle, how would I know to investigate something?

I'm not saying that there has never been malicious intent on behalf of mechanics or assemblers, of course there has. My point was only that mistakes do happen. We're only human!

Glad you found it. Rattles drive me crazy too. Friends sometimes comment that they cannot believe my 38 year old daily driver (145) is rattle free. This comes from people with cars that are only a few years old.