typeiii

The noise comes whenever the HVAC unit is on, regardless of whether the AC is engaged... And it's independent of engine speed. As a matter of fact the noise is there even when the engine is off. I was able to trace the noise to underneath the hood, and it appears to be coming from this thing, which I have no clue what it may be.

Tump43

That is the coolant overflow reservoir. It is unrelated to the air-conditioning, but its contents are pressurized. Perhaps it has a small hole/crack.

typeiii

Hmm, then maybe the vibration is coming from somewhere else... I'll be taking the car into an indi tomorrow, will keep you guys updated.

thefisch

The HVAC intake is on that same side but just under the windshield. You could be hearing the HVAC motor. You can remove the plastic cowl that sits over the battery and inspect the topside of the fan attached to the hvac blower motor. Perhaps something is stuck in there creating this sound.

typeiii

So as it turned out, it was from that little Bosch electric motor. Went and got a new one and guess what... SAME NOISE! I was like oh what the heck I got the one in a (few) thousand defective part from Bosch? Fine, got another and again... Same noise!

At the same time, I realised that no matter what temperature I set my AC to, I don't get any heat... (not easy to notice this when you live in Hong Kong)

So we now suspect that the motor is actually fine, but it's spinning dry. Will need to look further into the entire system to figure out what goes where and why this is happening because I refuse to believe that I got two bad motors from Bosch... It's gotta be something else that's causing the motor to spin loud.