So after a 358sbc swap in my 86 TA, there's a rattle that we can't seem to find happening at low speed acceleration until around 2500rpm. It's not spark knock, I can see that on the datalog and it's definitely not that and it doesn't seem to be an exhaust leak either. Is there anything that people can think of to try? Maybe the handbrake cable being a little loose or the u joints? Haven't checked those but I have had the driveshaft out very recently, and it doesn't seem to change depending on speed. I have gotten under the car and shaken everything around on a lift and still can't seem to find it.

Hmm, take a video? Parts of my AIR system would rattle against stuff after I vigorously persuaded them, went away after I removed it all.

One of the most common problems is exhaust or headers hitting the car as engine moves

I just found a noise in my 91 TA vert that drove me batshit for weeks. When I would drive slowly either in forward or reverse, up to about 20 MPH, I would hear a "clanging" sound from under the car, as if I was tossing a quarter into an empty pot on the stove. It disappeared after around 20 MPH.
I checked everything under the car multiple times. Pulled the wheels off. Long story short, finally found it.
Aluminum driveshaft, and either a piece of welding slag, or a rivet tail from installing a balancing weight had broken loose, and was clanging inside the driveshaft tube.

Recently had a cracked/broken weld on my truck exhaust start to rattle off n on. The sides fit like teeth and both ends are tight/mounted good, so it wasn't really noticeable until I pushed em apart.