My phone creates static noise when WhatsApp is open in 8.1

I haven't noticed this with any other app except WhatsApp, but since I updated to 8.1 my phone makes a soft (but audible) static or humming noise sound when WhatsApp is in the foreground. It's a bit like my phone has a fan on the inside to cool it, or the noise that speakers make when they're powered on but don't produce any sound.

Can anyone try this out for me? I have a Lumia 920 with the following versions:

OS: 8.10.12359.845

Firmware: 6051.40000.1349.0004

Hardware revision: 6.5.0.1