There's fluid on the shock body visible between the spring coils; it appears to be leaking. Forgive me if you know this, but 4th Gen Sport Editions have X-REAS, which is a system that pipes fluid in an X pattern (hence the X in the name) between shocks on the opposite corners of the vehicle. So, front DS to rear PS and front PS to rear DS. This provides better cornering and so on, as the Toyota video below explains (it shows a 5th Gen but the system works the same).

But if (well, when) the shocks start leaking, you're done. Toyota "recommends" replacing the entire system, which costs many thousands of dollars. I've heard the shocks can be replaced one at a time but it's very expensive and not worth the bother since it won't work correctly if you get a leak somewhere else.

FWIW, I took a very conservative course when replacing mine b/c I didn't want to drop a grand or two on a setup I wouldn't know if I liked until I drove the truck. It's not a shirt you can return if you change your mind. FB Marketplace usually has Tacoma take-off suspensions, which are a great choice to start with. The 2nd and 3rd Gen Tacomas are plug-n-play on 4th Gens except for the lower rear shock bushings, which are easy to replace. The link in my sig explains everything, including how to remove the X-REAS (you must follow the process b/c the system is pressurized).

Anyway, my first Tacoma setup was from a 2015 TRD Offroad and I bought new FJ springs for the rear to go with it. Total cost? $200 and a day of my time to swap it in. That gave me time to get a feel for what's out there. I upgraded to a 2020 Tacoma setup for about $300 more a couple years later and have now decided to go with Eibach. (There's lots to like about them but that's another discussion.)

There are a lot of X-REAS problem threads and I encourage you to do more research, but if that shock is leaking, that side may be bottoming out and that's making the sound. It's not 100% certain but it's pretty darn close IMO.