Occasional hum

[Barry]

This is very similar, if not the actual case - https://www.audiophonics.fr/en/diy-e...l-p-14211.html

looking at the photos, all surfaces appear to be anodised. Thus you will need to re-tap the holes, and re-countersink the holes on the external surfaces. It is almost certain the fixings are metric (M3), so I would replace them with un-anodised steel screws. This should solve the problem

Ideally I would remove the anodising from all mating surfaces, but the above should be sufficient.

Good luck.