This happened to my 88 735 a couple of years ago. It turned out to be the plastic supply pipes to the heater core. The molding seam in the pipe split. Part was ~ $20. Replacing it requires removing the dash, center console, etc., not too complicated but time consuming and tedious. I fixed the broken mounts on my hvac motors with wire and epoxy while I had the dash open. I think there was a recall to adress this problem by putting a pressure release coolant reservoir cap on. Good luck.