Hello Eddie,
I certainly don't get any screaming after turning the Input to 'None' and Output to 'Mac Speakers' which I will try should this annoying sound recur. As it stands at the moment I shut down a Dorico music programme, which I use for composing, and the screaming disappeared. It sounds very much like the plug-ins used in the Dorico are not compatible with the Logic sounds. I have to admit I am no expert and as I don't need both programmes open at the same time this seems to be the solution.
Many thanks for your helpful input. Like most of us Logic's sounds, in-built and from outside sources, take some beating and that is why I transfer compositions from Dorico to Logic.
Cheers,
J. Barry.