I'd be willing to bet that your noise is the monovalve. Does it sound like its coming from in front of the glove box? I had one of these kick my butt when I worked at the dealer and finally I replaced the monovalve. This still did not fix it but it was clearly the monovalve. My shop foreman had me mask the noise by stuffing insulation between the glove box and heater case. I didnt like the idea and wanted to FIX it, but he was the boss.
Mercedes Benz Master Tech
ASE Master Technician
Owner of "Behind the Star Inc."