You mean to fill the void in the frame?  Like expanding foam?

maybe foam, not sure if it would damage the carbon.

Either that or some sort of glue that the object would stick to.

If the calliper mount holes aren’t actually ‘blind’ and do give access to the internal void I’d consider a heavy spray grease or wax rather than an expanding foam.

If it’s a small object that’s rattling it will adhere to the grease and stick.

Of course, carbon frames are big echo boxes and the noise may not actually be something in that particular tube...

Not interested in finding out what that metal object could be before you start ignoring it?

thanks. I think the holes give access.

mattbee - maybe something like this?

https://www.toolstation.com/shop/p41925

Best option to stop the rattle is, providing you can reach, to inject a small amount of silicon sealant into the area of frame and let the object catch on it. Then leave to dry.

Just pick a low modulus non acetoxy sealant.

It may just be some loose carbon from the manufacturing but personally I would want to find out what it is. A small piece of something solid would make quite a rattle.

I used a bit of expanding foam. Sorted it.

I had the same thing in the carbon seat stay of a Cannondale Scalpel years ago.  There was a small access hole in which I dripped some strong superglue.  Just tipped the bike backwards and forwards, left it for a while... no more rattle.

I think the object was a bit of rivet from the brake mount.

thanks for all the suggestions!

It may just be some loose carbon from the manufacturing but personally I would want to find out what it is.

How would you find out what it was?

Yes, no way to find out what it is.

i've has another look at it. I think the object is the back of a rivet from one of the cable guides. Also no access from the disc mounts as they are closed off.

There is a small threaded hole near the seat tube junction where i might be able to spray some liquid wax and let it drain down the seatstays.