Take out the QR skewer so we can rule that out ?
Removed QR, spun by hand and still there. So not to be beaten, tyre off again, rim tape off again and gave the wheel (without a QR in to eliminate) a damn good shaking and yes the rattling was still there. A bit more shaking and the rattling stopped.
Whatever it was must have worked itself out. I can only assume something lodged between the inner and outer skin of the rim.
If I'd have had some good old fashioned Pipe Cleaners I could have "rodded" between but alas I haven't!
All spoke nipple heads are intact and not loose so gave them a little 3 in 1 whilst rim tape off.
Tyre back on, wheel back on bike, quiet as a mouse.
The moral of the story, if at first you don't succeed shake the wheel some more 😊