clunking felt in the gas pedal after driving about fifteen minutes

The place to start looking is at the exhaust pipe hangers. The rubber isolators soften when they warm up and will allow more flexing. A pipe may be rubbing against a heat shield or metal bracket.

I do not know where the anti-sway bar inner bushings are located, but if they are on the engine-side of the firewall, see if the clunking stops when you turn the steering wheel as little as a quarter turn either way. If the noise stops, suspect those bushings. When turning, even slightly, one wheel moves down and the other moves up, thanks to an alignment angle called "caster". Those movements put a twist on the anti-sway bar, and pressure on the inner bushings. If the noise stops, you can verify this with the car on a drive-on hoist. Wrap your fingertips lightly around the anti-sway bar, then bounce the car up and down just a little, by hand. You will feel the looseness and thumping between the bar and the bushings.

You an get the same thumping with other anti-sway bar mounting designs, and on some cars the outer ends use rubber bushings instead of the more-common links. The hole through the center of those bushings can become worn larger and allow the bar to bump up and down inside it.

Your car uses outer links with a pair of ball and sockets on each one. Those will cause rattling too, but those are not normally affected by time or temperature.

There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.