Car too loud due to previous owner fine?

Hello. I was stopped by the police, and they said my exhaust was too loud, but they let me continue because the dB meter was broken.

If the exhaust is indeed too loud (because it's worn or tampered with), how am I supposed to know? The car passed its MOT immediately after I bought it, and I haven't changed anything. The previous owner didn't tell me anything about it, either. I'm not a professional, and I don't have a measuring device. It doesn't seem particularly loud to me, and I don't notice any unusual smell (which would supposedly be noticeable when the catalytic converter is empty).

I'm familiar with the saying "Ignorance is no excuse," but I find that very difficult in this situation.

Incidentally, it's a BMW 325i E92 3L. It's the typical cliché car that's constantly being stopped. It's fitted with an original BMW performance rear muffler, which is approved for this car without registration. Of course, this could also be tampered with or defective. But again:

-How should I know?

-Can I be penalized for this despite my ignorance? Fines would be annoying, but not as bad as points, since I'm on probation.

Thanks in advance.