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It's a Bambu P1S. I have never used one but first job I sent to it was this. Was a big hit in the office.

That pretty much confirms it's inside the trans. Though, when I had 2nd let go on one, it broke off an entire tooth on the small gear only make a light clicking noise, and lodged the tooth in the center section. Draining didn't find anything, only taking apart showed the issue. Though with how old yours is, it might also be loose bearings causing enough slop to make that constant noise

Have you drained the gearbox oil and see what comes out with it? Also carefully feel around inside the gearbox through the drain hole for any other fragments that may be present. Noise will travel to different areas. You’d need to use a dowel/long screw driver/stethoscope to pinpoint where the noise is coming from more precisely. Seems more likely that second gear is chipped, possibly both gears - if that’s the case you’d get an un-rhythmic sequence that will repeat after some time. This is because all gearsets have at least one gear that’s got an uneven number of teeth so they wear all teeth against each other for even wear across all the two gear cogs’ teeth.

For functional and tough parts my go-to is Armadillo by Ninjaflex. It's a very high hardness TPU so it acts like a normal rigid polymer but it has a very high bond strength between layers. It takes some tweaking to get your settings dialed in and may require dual-extrusion or a multi material unit when you print support material. Since it bonds so well to itself, getting supports off without cutting and sanding tools isn't easy. The best overall bed adhesive that works with almost all polymers, that I've found, is normal white school glue spread with a damp paper towel (3 quarter-sized dollops spread thin). Once that dries, lay a layer of Magigoo PC bed adhesive over that. That coating will last 3-10 prints before you wash it off with dish soap and reapply. I've wanted to print a cup holder. Right now I'm just using a cardboard drink carrier I got from a coffee place on my passenger side floor board.