When i have being reversing my car e.g. into the drive a rattling is present some times. i have found out that it can be that the gear could be out of alignement, or their could be a chipped tooth on the gear itself.
(95 cinquecento).
leiws

or it could be heat sheild hitting exhaust, try bending that first. see if it still makes the noise

if it was a gear problem, i'd expect it to make the noise all the time, not just sometimes

Mine does that - it's the flexy pipe too close to the heat shield
John

My Flexi ratttled on the body

I replaced it with a Grp N . It got worse, but sounded better. Oh Well...

mine does it when im reversing on the drive think its the heatsheild on the exhaust

Would have to agree with it being the heatshield. I ended up jacking my car up & 'manipulating' the shield into a more acceptable shape!