Hi Martin W
I read your comment that you do not have a NVH kit.
Question : How is your bevel-box then attached to the frame?

When you use a PB-bevel-box-mount, than I would advice to read the article about alignment in Bruces his m3wbevelbox.work website.
It is an article valuable for all M3W belt-drives
Possibly the article will give a solution to your problem

link : Being good aligned is an issue to consider

One ant does not know anything, 2000 are a super-intelligent brain. That's like this helpful forum.