So my 93 Del Sol Si (D16Z6 with S20 trans) started making noises about a month ago. It began while driving to get dinner, and the noises started about halfway through the drive. From what I remember, there wasn't anything that caused this, I didn't rev the **** out of it or anything, it just started. Sounds like a metallic rattling or something. Got in and drove home, noise was there when it started, but went away shortly after.

Stayed quiet for a couple of weeks, then it happened again while driving to work. Again, about halfway through the drive. Car sat for 8 hours while at work, and the noise was still there when I got back in. Stopped at a gas station to check the oil; seems low so I refilled it. Noise stayed until I got home.

Now, I didn't notice any drop in performance while this was going on. The noise seems to be constant from idle to WOT, and rises or falls with RPM. I just did a compression test, here's the results:

Cyl 1: 160
Cyl 2: 156
Cyl 3: 171
Cyl 4: 162

So, not ideal, but nothing crazy. I did notice there seems to be a bit of oil fouling on the #3 spark plug, so maybe some burnt-on oil and carbon causing higher compression?
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I also checked the cam timing, and it seemed to be retarded by a tooth or two. On that note, upon visual inspection of the cam and valvetrain, everything seemed to look okay. I have since remedied the timing, though I'm sure I should also replace the belt soon. I should probably double check the timing since the engine has run since then. The noise was still there immediately after resetting the timing.

Now, after the compression test, I ran the engine to try to get a video of the sound. The sound was absent for a minute until the engine warmed up, then it returned. Linked a couple of videos to the sound.

https://vm.tiktok.com/ZM8RBTsu2/ (forgive my basic-bitch Tiktok link, I couldn't upload it anywhere else on my phone).

My next step will be taking off the oil pan to check the rod bearings. Any other thoughts or suggestions?

 

The higher compression on 3 could be because of oil sealing it up, which also correlates to oil fouling of the plug in #3. Your rattling could be broken ring lands or it could be piston slap from seized rings. Or as you are implying, could be rod knock which your bearings would tell you. Can't say pulling a crank cradle to get to rod caps is any fun under a car. I don't like to do bottom end work with the engine in the car, too much pain and struggle in my opinion.