https://www.theverge.com/2017/10/26/16555460/google-pixel-2-update-clicking-sound-fix

Google says it’s prepping a software update to fix the faint clicking sounds some Pixel 2 units have been emitting since the device and its XL variant landed in consumers’ hands starting last week.

“We are validating a software update to address faint clicking sounds on some Pixel 2 devices. The update will be made available in the coming weeks,” a community manager wrote on the Pixel User Community forum this afternoon. “The clicking noise being reported does not affect the performance of your device, but if you find it bothersome you can temporarily turn off NFC in Settings > Connected devices > NFC.”