Buzzing Noise on Logic Pro with Input Monitoring

Whenever I try to record audio, and use input monitoring, there's a quiet but extremely irritating, and constant, buzzing sound. It stops as soon as I switch off input monitoring. It happens whether I'm using headphones via my scarlett solo or my macbook's headphone jack, or whether I'm using the mac's internal speakers. I can't record anything with it happening because it's literally just buzzing over everything I do, and if I don't use input monitoring, I can't hear myself sing!

I've tried experimenting with the latency/buffer size, and that hasn't worked. I've tried restarting Logic and the scarlett solo, to no avail.

It only seems to happen when I'm using my mic, rather than the macbook's internal microphone, which could suggest it's a mic issue. If that's the case: what do I try for this!? How would I know if it's the microphone causing the problem?

Any suggestions?