EllisoJo

Deepfat's idea of checking again while your TF is on a ramp, moving it slightly to stimulate the noise while you stand underneath sounds like an excellent idea.  I was wondering if you had investigated Joulian foulger's suggestion: Ref. https://www.the-t-bar.com/forum/9-mgf-tf-pitstop/97507-undetectable-rattle-at-the-front-offside#196075?  A bar behind the dash, loose at one end is a possibility, and would be more difficult to hear UNDER the car?

I have investigated inside the driver's door. The rattle is now different, but still there, despite adding foam padding behind what I THOUGHT was the culprit!  At least I know there is no rust inside that door, and I sprayed Waxoyl over the lower skins and anti-intrusion bars.
Good luck with your continuing search
John E