https://www.theverge.com/2017/11/20/16678620/google-pixel-2-buzzing-sound-phone-call

Google is releasing a fix in the coming weeks to remove a faint buzzing sound that appears during phone calls on some Pixel 2 devices. A community manager posted a short message on the Pixel User Community forum confirming the fix, stating, “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.”

Some users received replacement handsets, but said the problem remained. Others described the noise as a hissing sound. The fix comes shortly after the company issued a software update to fix the faint NFC-related clicking sounds some users experienced last month.