I have noticed an intermittent loud 'thunk' noise from the front/dash area, seemingly only when the heater is on. It is fairly strong, at least to me, akin to the sound of what warping sheet metal (under temperature changes) may do (or a big spring unloading). Sometimes, there has also been a transient vibration, as if I have an ICE engine/transmission in front of me. Anybody else with the same observations? Anything to worry about?