Audi A4 AFN engine knocking at 1800 rpm

Hello.
Today, I noticed something strange with my car. When accelerating, I hear a very faint rumbling sound in each gear, around 1800-2000 RPM. There's nothing before that RPM range, and nothing after it. If you keep the engine at that RPM range in neutral, the rumbling is still there, but you have to listen very carefully to notice it. It's more noticeable when driving. To me, that sounds a bit like an exhaust system.
Perhaps someone has encountered the same problem before. I think the structure is the same for the Passat and the A4.

Regards,

Thomas.

"On our A4 (AFN, manufactured in 1996), the catalytic converter has been loose in its housing for a few weeks. It's getting louder and louder, but I suspect it won't be long before my wife (who drives the car mostly) forces me to replace it."

You can hear the noise at a specific RPM, I think around 1500 RPM, coming from the entire exhaust system when you're underneath it. To be sure, I drilled a small hole in the catalytic converter housing and screwed in a sturdy metal screw until the monolith was secure again. It only worked for a short time.

However, the air filter housing is also a common culprit.

Best regards, Rudi.