Campagnolo Shamal Ultra G3 front wheel rattling noise?

I have an ongoing issue with an annoying rattle coming from inside the rim of the front Shamal clincher wheel. It does not appear to be coming from the spokes or valve hole. When I remove the tyre and inner tube and shake it with hands in the "quarter to three" position the rattle remains.
If I rotate wheel place my ear close to the rim while rattling the noise always sounds wherever my ear is located, ie it doesn't appear to be coming from one particular location but from the whole rim. Can anyone help?