It definitely sounds like you're suffering from something periodically scanning for WiFi networks. This seems to be a common issue, which gets worse because it's currently done quite inefficiently in the driver.
The main offender seems to be NetworkManager, looking for better access points for roaming purposes and I've seen a few tips on how to make it not do this, but I have not tested these.
If you feel adventurous you can try out my experimental patch that speeds up the scan, it seems to work but I've not done enough testing. You can find it here, https://patchwork.kernel.org/patch/9085741/ and it applies to a very recent kernel.