I recently picked up my MYLR with HW4 and notice the same. It sat in my garage for a week with that buzzing sound 24/7. I did't think this was normal so I scheduled the earliest appointment available at the Tesla Service Center, but it was a week out.
I wasn't going to sit around and watch my new car self destruct so I decided to remove the frunk & HEPA filter to see where that noise was coming from. I traced it back to the Battery Coolant Pump & Powertrain Coolant pump both was staying on. It didn't matter whether the car was charging or not, plug in or not, day or night, inside or outside the garage. The temperature at the time was between 65F-75F degree. Nothing I could do to make those two pump turn off except to disconnect both the low voltage and high voltage battery. So it sat for a week with the batteries disconnected until I got it to the service center.
Got a massage a day later letting me know that my vehicle was ready for pick up. Its almost impossible to get someone to talk to you on the phone so I went to pick it up. This is their official finding, "Checked alerts, no alerts are present at this time. Checked level of coolant, coolant ok at this time." They told me that there wasn't any fault code in the system therefor there is nothing wrong with the vehicle. The vehicle is operating as designed/to specs. Noticing I wasn't getting anywhere with them at the service center, I took the vehicle home the same condition I brought it in, with both coolant pump still running continuously.
I still didn't feel comfortable with both of the coolant pump running nonstop. So I decided to dig around some more. I deleted my phone as a key and waited for an hour to see if that would help, nope. Turn off Wi-Fi, turn off charging schedule, turn off Bluetooth. No, no and no. So I decided to do a "Factory Reset" and let the car sit for about an hour. To my surprise the pumps stop running. I than went back in and turn on the options I wanted on one by one (waiting 30 minutes in between because any time the vehicle is awake the pumps will run for a while) to see what it was that trigger the pump to stay on continuously. It turns out to be the Sentry Mode. I can than turn on any and all other options/function I wanted and the pumps will only run for a short while than turn off. As long as I keep the Sentry mode off.
I know this is not comparing apple to apple, but a friend has a M3LR that he picked up on the same day as I picked up my MYLR and he doesn't have this problem. On his M3LR with HW3 you can turn the Sentry Mode on and leave it on. The pumps will turn on for a short prior of time than off until there is an activity on or near the vehicle.
I don't know if this is a Model Y issue or HW4 issue. For the time being I'm keeping the Sentry Mode off and keeping my figure crossed in the hope of some software update is in the work.
Sorry for the long post.
