Just set a brand new Stans Arch mk3 up tubeless,put the milk in and gave it a shake and noticed a rattle in the rim opposite the valve. Tape off , had a look and I think it’s the brace where the rim is welded that’s come loose. Is it structural/defective or ok to gob a bit of silicone in it?

The heat sink on Stans is fairly notorious for falling off. I would send it back.

Apparently not an issue, no need to send it back. Direct from Stans website:

Rattle noise in rim
You may have a loose weld sleeve. This is used in the weld process as a heat sink. It is crimped into place with a dimple in the material being visible at the weld junction. Occasionally the crimp does not hold and the plate comes loose. It is not a structural element of the rim so the best solution is to remove the rim tape and apply a little non-water based glue between the weld plate and the rim. Tubular cement or super glue will do the job. All rim makers use a weld plate and run into this situation on occasion. Please call NoTubes if you need yellow tape to reapply to the rim or if you are not comfortable gluing the weld plate on your own.

Had this with a rim before, not a stans, forget which though, the rattling was doing my head in until I worked out what it was and glued it up!

Mavic used to do something similiar, it was irritating but not troublesome.

This happened on both my Pacenti rims, glued them and it’s been fine since.

As above – I had exactly this with Hope Hoops. I stuck the heatsink to the rim with whatever glue I had lying around and it was fine (until I cased a landing and pringled the rim). At the time I complained to Wiggle and they sent me roll of yellow tape foc.