https://www.cnet.com/news/google-pixel-2-buzz-update-coming-in-weeks/

A Google community manager identified as Orrin shared on Google's Pixel user community forum that an update is very much in the works. He wrote: 

We're rolling out a software update in the coming weeks which eliminates a faint buzzing sound on some Pixel 2 devices when the phone is placed to your ear during a phone call.