I’ve just had exactly the same issue with mine. Slacked the RH belt tension from 3mm to 4mm - problem solved! Good luck with yours.

Hi all, Long time reader of this excellent resource, thought I'd finally break cover and introduce myself. I have a 1987 GL1200 Interstate, and I love the thing! I'm currently working my through the usual maintenance list, and preparing for lots of touring when things settle down on the Covid front ...