I connected a Sphere to an ISO Offset to create a fog volume then connected this to a VDB Convert and set Convert To: VDB.

Since I know the Cloud Noise node requires a VDB format this was supposed to work when I connected it to the first input, however, the fog item just vanishes. It works fine if I use a VDB from Polygons node instead.

Is it a glitch or am I missing something?

I think the issue here is that the 'ISO offset' SOP creates a standard Houdini grid volume, and I think the 'VDB convert' SOP requires a VDB input - which it's not getting. Although, when you look at the node info for the 'VDB convert' SOP it does suggest it's outputting a VDB...

EDIT:
Scrap that. Having had a quick play, I can see the issue: The isoOffset generated volume doesn't have a name attribute. Just add a name SOP - with a name of 'density'.

Thanks, that does the trick.