Getting a low rumble specifically when the climate is on, with the air flowing only through the defrost vent. AC off does not change this. Goes away if I add floor vent or chest level air flow. Before I'd had time to figure out the above, service saw the car and thought it might be AC cable against an inner cabinet, and opened all to be sure no contact was being made; given the above this obviously was not helpful.
I should add that, when air is sent only through the defrost vent, with or without AC, whether fresh air or recirculating, I get not only this really annoying low rumble as mentioned, but also a fan noise up high (dash, from the defrost vent in the middle). Of note, this is all new in the past week.
Thoughts before I bring Elwood back to Service?
The low rumble issue has been a personal quest of mine as it really bothers me (A big hobby of mine is related to sound engineering and AV stuff). I believe the low rumble (beyond the normal tire and suspension noise over road surfaces and less than perfect road surfaces) actually is emanating from two primary sources:
1) The rear hatch lid
2) The rear seats (not the kid seats) themselves
I know exactly what sound you are describing with the HVAC system. The best way to describe it is it's like tapping the head of a bass drum lightly, you kind of get a 'whump, whump' type sound. If you tap the rear seats from the back (so stand behind the car with the rear hatch open and tap the back of the seats) you can duplicate the sound. Also you can do it from the inside of the car as well by tapping the back seats while reaching back from the driver or passenger front seats.
I found it strange that the sound would still resonate from the rear seats in relation to the HVAC issue even when the windows were open ands the sunroof was open, but what might be happening is the defrost vent pushes the airflow in just the right way to resonate the rear seat covering just enough to act like a bass drum head. It doesn't take much movement of a resonate surface to cause a low frequency rumble.
The temporary solutions I've found fairly effective at mitigating the problem:
1) Adjust the rear hatch rubber shims (extend them to the point where the rear deck almost won't close) to tighten the pressure on the rear hatch between the car and the hatch. The shims are on either side of the rear lift gate that raises (in the left and right corners by the tail lights).
2) Adjust the round flat plastic knobs that the rubber shims sit up (extend those out)
The above two fixes will primarily take care of the whump, whump sound when you are driving, but they really don't seem to help the HVAC issue that much.
I think the HVAC issue may have to be addressed by:
1) Putting additional padding material in the rear seat with the goal of preventing the resonate surfaces in the seat from vibrating
2) Improving the tightness of the latching mechanism so there is absolutely no flex in the rear seats.
3) Use a different material for the rear seat that has improved low frequency sound absorptive characteristics.
4) Change the airflow of the front windshield vent (This particular issue is being looked at by Tesla closely in regards to the fogging issues on the front windshield)
5) Install the rear parcel shelf - That would have the effect of making the speaker cabinet (rear hatch space) smaller, and potentially change the frequency of the resonance to something in the higher frequency range which might be less annoying
Another potential measure that owners could take would be to look at using some bass traps (from the audiophile world) in strategic locations in the car. Basically address the problem like you are having unwanted bass resonances in your listening room (in this case the car cockpit would be your listening room). It does seem a bit extreme, but it would be a way to reduce or eliminate the low frequency issue.
Also upon reflection, I think part of the issue is that the rear hatch is acting like a bass cabinet. The clearance between the top of the rear headrests and the top of the roof/hatch is relatively small, which essentially creates a massive speaker cabinet in the rear hatch with the rear seats acting like the bass drum head. Any movement of airflow in that space will act like a speaker. So if the hatch moves, the airflow movement will resonate the rear seats and if the rear seats move because of the airflow in the cabin (with the defrost vent moving airflow around the top of the car and into the back hatch) it will act like a speaker cabinet and resonate.
Hope that helps your quest to get a quiet cabin!