Kind of hard to judge the difference between a click and a thunk long distance, but it sounds to me like the valve is OK. I would call mine a soft thunk.

Look at the plumbing. You will see how the lower hose (1) runs to the auxiliary pump, then a short piece (2) to the monovalve body, then a piece (3) to the heater core. On the left side of the blower cover there is another hose (4) that returns the coolant to the engine.

Leave the wiring to the monovalve unhooked, run the engine for a while, turn the heat or defrost on, and see if you are getting hot water to the heater by feeling hose (3), and also check hose (4).

This will tell you if there is a problem with water not getting through the system and if not, where the blockage is.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe