Here's a theory in response to Suginami's post. The vacuum pod for the defroster flap is mounted on the right side of the blower box, just behind the glovebox (at least on my 89 300E). I had a similar sound that "sort of" sounded close to the center vents but more on the right. Eventually it stopped. I took apart the vacuum pod, which is dual port and has two rubber diaphragms. One of the rubbers was totally trashed. That pod actuates when either the A/C button or the economy button is pushed to direct airflow to the center vents. Could be I was hearing the diaphragm slowly faily and flapping around from the vacuum.