Its your thermostat cover and it sounds like your diaphragm might have a hole in it.
You need to pull up a exploded drawing of your engine or look through the manual. Those motors have like 7 or 8 parts inside the housing ie springs, poppets, plungers, diaphragms etc.. Its not a one piece deal. More than likely its your diaphragm that is leaking. Not so much a sign that the thermostat is going bad but it is a sign that the diaphragm is bad.