When i first bought my 84 380se the hvac would not blow hot air
traced problem to water flow control valve. Replaced guts in valve
and got hot air but then i noticed a thumping noise while sitting still and no other noises out side car. If i remember correctly it was coming from the the valve and or electric water pump for hvac
Since it isnt very loud at all i never worried about it and hvac has worked fine for the last two years "knock on wood" lol. But im driven a totally diffrent machine the you guys.