The solution advised by @Rugby worked for me too, so I’ll set it out here in case anyone else has a similar issue.
The problem seems to be with the RealTek HD Audio device driver.
You can uninstall that driver in the device manager under “Sound, video and game controllers”.
If you check the box to delete the driver software upon uninstall, then Windows will restart and automatically install the native Windows HD Audio device driver.
You should then be able to use exclusive mode in Roon for hi res files, without the white noise !
If you decide that you want to re-install the RealTek HD audio device for any reason, you can find the latest version at:
http://www.realtek.com.tw/downloads/
I am hearing a crackle using this to listen to hi-res in Exclusive mode. Will post back if I can track down how to get rid of it.
Edit: Crackle disappeared after I toggled Event Mode off and on. Just gremlins.
Thanks Rugby, now officially cooking with gas and no need for ASIO4ALL at all at all.
Cheers,
Andybob