'A' frames don't so much rattle when worn but make a clonking noise especially on acceleration. To rule out the 'A' frame give that a good wiggle about to make sure things are still solid.
Rattley noise i'd be looking for something loose in the boot to start with. Perhaps if you have an alloy boot floor pan this worked loose and is vibrating a bit ?
Just recently i had a rattle on my car, wasn't sure where it was coming. Sounded as if it was coming from under the dash. All made sense as only last year i replaced my oil px gauge so things where disturbed a bit. Anyway turned out it was my alternator and the lower rear mount lug letting go. Amaizing how sound travels.
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Gordon.
Morgan LeMans62