If it's a sudden jump straight after the Jarring noise and it's just on the Long axis then I suspect it's the ballscrew end bearings floating.?
Grab the gantry firmly at both sides and try to move back n forth. If you have end float you'll feel it clunck.

If it's losing position over time then I'd suspect a lose coupling first, after that then I'd look at the wiring. These cheap machines come with crap wire on the motors which gets brittle and breaks internally but still works and causes all sorts of strange things like this. Same with the connectors there rubbish and need replacing.

From you saying it works fine then does this then I don't think you will have it over tuned or anything so wouldn't go messing in that direction just yet.

With the noise your saying it's making then 90% chance it will be mechanical 10% wiring or electrical.