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Identify Sound Heard While MX Is Asleep

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Went into garage while car was asleep (and not plugged into HPWC) and heard a sound (more like a hum) coming from front area of car. Could this be the 12V battery charging? The car has been idle for 4-5 hours - how long would it normally take to recharge that battery? What sort of operations would tend to deplete the 12V battery?

If not the charger, what else might it be? Are there pumps or something that run while the car is asleep?
Could be a couple of things.

1. Battery temp. The car can cycle to keep the battery heated/cooled depending on where you are.
2. Overheat Protection - If it gets too hot, it's the HVAC turning on to keep it within the limits.
3. Smart Preconditioning - Since the last update, this has been a little buggy. We've found on occasion it's been turning itself on at random times. Just a couple days ago we lost 100 miles of range over a 24 hour period by letting it sit in a garage because it kept cycling on. Normally we only lose 10 miles at most overnight.
Depending on setting and software version, the charging of 12V battery may take hours or forever (trickle charge), but I don't think charging makes hum sound.

If I set Energy Saving off, I can see several hot spots on my X using thermal camera:
1. bottoms of the FWD
2. front camera assembly
3. instrument panel
4. two relays in the front fuse box:

These relays are K111 (on the left) which is for HVAC Rail and K101 which is for HVAC Rail group 1.