msmcleod
Alex Bull from Focusrite posted a link on AudioFanzine:
 
https://en.audiofanzine.com/audio-interface/forums/t.10452,scarlett-focusrite-2i4-help.html
 
I'd try msorrels' suggestion of increasing your buffer size first though, before reverting your drivers.
 

Thanks for the link :) Yeah I've tried a number of different buffer settings already, unfortunately to no avail

Anderton
Maybe a really short audio loop is enabled?

Hi Craig - no, it happens regardless of loop being enabled/disabled. When it occurs, CBB visually appears to play back: the Now time advances normally, its just the sound that's busted

Couple of tech questions
 
AMD or Intel system?
Is the device connected to a USB 2.0 or 3.0 port?
 
My friend had a 1st gen Scarlett on an AMD system into a USB 3.0 port and after the latest driver updates a few months it was nothing but grief for him.
 
 

What I was seeing sounded like the computer hadn't finished setting up the next ASIO buffer and the 2i2 driver "looped" on the one it just played.  Which because it's so short would create some awful noise/buzz/etc.  I don't think the 2i2 driver handled/handles recovering very well in this case.  The only solution was to set the ASIO buffer (none of the settings in SONAR had anything to do with it) to a much larger number (1024 samples I think).  Which made the buzzing stop but of course made recording/playing pretty close to unusable.
 
The 2i2 does seem to have some issues with the power supply/USB circuit as well, but it wasn't the problem.  I wasn't impressed with the isolation between the USB port/power and the speakers.

msorrels
What I was seeing sounded like the computer hadn't finished setting up the next ASIO buffer and the 2i2 driver "looped" on the one it just played.  Which because it's so short would create some awful noise/buzz/etc.  I don't think the 2i2 driver handled/handles recovering very well in this case. 

This is exactly the symptom.
 
After checking, I actually had the 4.36.4.484 driver installed already, so I updated to 4.36.5, but saw the same problems.
 
The Scarlett was connected to a USB 2.0 port on the back of my case (next to other peripherals). Then I tried plugging it into a different USB port (this time a USB 2.0 port on the front of my case) and this makes a HUGE improvement. I might get the problem once or twice in a 5 hour session now :-)
 
So I guess it's either the USB cable is on its way out, or is somehow interfered with by something else I've got plugged in..? Still can't really be sure, but I've ordered a new decent quality cable to see if that eliminates the problem completely.
 
Thanks for your suggestions