macbook pro shutting down

My 2020 Macbook Pro is shutting itself down multiple times per day. It seems to happen when I walk away and leave it for a bit, or shut the screen. Maybe it's something to do with trying to go to sleep? A day or two ago it was making a new sound - like a sigh, or release of steam sound.

I tried resetting the SMC and the PRAM and it doesn't seem to help. It's running Big Sur, and is up to date.

Thanks for any advice!