cleaned the bike and fitted some old XTR callipers in place of the SLX’s i had,

However they are howling,

Can anyone give me some reasons that they could be making a racket and what to do to shut them up. GT85 on the disc is not really my favoured solution.

Contaminated pads?

Change them and see if the problem goes away.

You might be able to clean the old ones but I had no luck.
In my case it was the SLX caliper leaking onto the pads. Problem solved by changing brand of brakes under warranty. 😥

put copper slip behind the pads

Actually, copper grease behind the pads is a very bad idea. Not designed to take the heat, will seperate and contaminate the pads.

What you want is Castrol Red Rubber Grease (or a similar derivative). You can buy it on ebay for pittance. Apply a little to the rear of the pad, and around the caliper to say goodbye to squeal.

or spray some cola on the rotors

HEY it works