Cubase 10.5 issue

Question
I’m getting a very loud feedback noise burst on all channels for about 3 seconds right after I do a mixdown.
This has only started happening recently. I tried rebooting my PC, checking any malware / virus issues, started Cubase 10.5 in safe mode , etc. I’m using a focusrite 212 3rd gen interface
Any suggestions ?
Thanks
Kevan