I still say it's a vacuum leak. It is happening under a low vacuum situation, when you open the throttle. There should normally be enough reserve in the vacuum reservoir, but any leaks will nullify that reserve. You should at least consider yourself lucky that you are not getting heated air in your case.

It actually sounds like it might be a bad vacuum diaphragm under the dash, or switch over valve, because the heater valve is actually open in the w201 in the presence of no vacuum.