I have an Sig p365X.

The spare 15 round mag rattles when I walk. As I unload it the rounds rattle until there are only 5 remaining rounds in the mag. When I reload it doesn’t rattle until I add the 15th round.

Is the cure a new spring kit?

Nature of the double stack beast. Some just do it with certain ammo or all ammo. I’ve seen it from Glocks to CZ to SIG to HK.

Try giving the spine a “whack” to see if that seats them tighter.

Thanks for the suggestion.

I’ve tried hitting the base on a table, the front and back of the mag against my hand, etc but to no avail.

Not a 365, but I have a number of Glock mags that just do this. I’ve found that with some of them, a seating tap every few loaded in helps. Hitting the mag too hard seems to have the opposite effect and the rounds will bounce off the back inside body of the magazine and rattle more, not less, so you want to get the right sort of dead blow mallet effect. Some magazines just do this, unfortunately.

"Double stack" or "double column" magazines really aren't. They taper to a single column at the top. Not all of the rounds are going to be under the spring tension all the time. Most rifle magazines are true double-column (except PCCs that take handgun magazines), but few handguns other than oddities like the Steyr GB are.

I have 3 15 rounders that kept rattling after walking around for the day and it was very annoying! I switched out the springs for the maggguts plus two kits. No more rattling been about a year so far. I would slap them hard to reseat them every morning before switching out the springs and have not done it once since!

I forgot to mention my 21 rounders do not make any noise even when I jog and I carry them a lot now.

^^^thats my plan too. Unfortunately mag guts is out of their spring kit for my mag so for now I’m on their wait list.