It’s a 2009 Honda Civic, LX Coupe 5-Speed AT, 1.8L L4 SOHC 16V.
The problem is a low, rumbling, vibrating sound, almost like an overly bass system or a muffler. The noise is most pronounced when the engine is cold at about 2000 RPM. But it never goes away completely. Also, the noise cannot be heard from outside of the car, but it’s very pronounced in the inside.
Here is a sound recording of the noise (listen on a powerful stereo system or with bass boosting headphones): CivicSOUND.wav
And here is a video (the audio quality is better in the sound recording though): CivicSOUNDvideo1.mp4
What could be the cause of this? Thank you in advance!