1989 560 SEL,Heating Problem

I have a 1989 560 SEL with olny 145000 km. When I washed the engine today, I noticed a problem with the heater. When I push the EC button, the heating works correctly. When I push the windshield button, there is a clicking noise under the hood. It is the same noise with any other buttons, except the EC button. Does anybody have any suggestions or ideas as to what may be the problem. Please contact me, thanks.