Stumbled on this thread while trying to troubleshoot my own buzzing sound coming from the passenger dash and thought I'd offer my root cause and solution. Did everything that everyone in this thread previously suggested but it ended up being the simplest of issue. If you pull off the dash cover and look underneath it, you will see that it is lined with very cheap and flimsy gray foam strips. Some of the foam was no longer adhering to the plastic dash, especially the ends of the foam strip which resulted in it flapping around and rubbing against the plastic dash. This resulted in:
- buzzing randomly, primarily low speeds because it reverberated more from road vibration
- randomly when AC is turned on but mostly independent of the AC being on or off
- when driving behind another car especially one with a deep exhaust or through a bridge or tunnel it'll buzz because of the reverberation in the air
Fix: pull off the old gray foam and basically re-align with a different high density adhesive foam strip (I just bought a random strip on Amazon and cut it down to the right width). Just be sure the new foam strip is not too thick otherwise the tongue will not be able to engage into the u-clips.