rapping noise after spark plug change

Could be a misfire, and might be cylinder 3 or 4 considering where you are hearing the sound. Did you disconnect the electrical plugs on the spark coils when you removed them to get to the spark plug down below? Maybe they aren't connected back up correctly, or connected to the wrong coil. To early maybe to get a misfire code, or you would need to get the codes read to see that. I would check connections and make sure the correct wiring harness plug is attached to the correct spark coil.

 

That's probably not on: I don't think it's physically possible to mix up the connectors; they're all very short, directly adjacent to the coil they connect to, and they have a positive locking mechanism.

I'm wondering if it's just the proverbial knock-on-startup, but that's probably wrong too.

All I can think for next step, and I believe someone else did this here with success:

put the old plugs back in.

 

I dont think its causing your knock problem, but the original Denso plugs Toyota installed in the Gen 3 Prius , have been superseded to a hotter plug. They are both iridium , but the new plug # , is hotter .
They were a Denso SC20HR11 from facory.
The new superseded plug is a Denso SC16HR11

You will likely only find these new plugs at a Toyota Dealer . I think mine were about $12. each !

june-2017-product-bulletin-supersessions-within-the-spark-plug-range

 

Could be a bad plug.

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There won't be any visible arcing since the high voltage path is about a half inch long down inside of the hole the plug sits in. And Mendel is right about not being able to hook up the connectors wrong because of wiring harness orientation. Impossible to get it wrong, and you only unhooked one coil from the harness. It wasn't one on the right front side facing the engine was it?

 

Same experience here after replacing the plugs with AGKs. But knocking stopped after 30 seconds or so. That lit the check engine icon. Then disconnected pos, 12v battery for 30 seconds. (Surprisingly did not reset radio?)

But was fine and danny after that.

Just asking as my knocking, Rapp tap tapping on my chamber door was pretty loud and alarming. Did you let it run or quickly shut her down?

Asking cause mine smoothed out pretty quickly but first instinct was to turn it off to check 2see what I mucked up.

 

More theories, given what Danny3Xd says. Would engine control ECU adjust spark timing for plug wear over time, and then misfire when given new, back in tolerance plugs? I let my dealer change my Gen III plugs recently at my 160K service with no ill effects. I changed my own Gen II plugs without incident. I don't think a 12V reset was part of either of those services. Hopefully all four plugs were installed properly, torqued to specification. A compression loss around a loose plug would cause a misfire. A code reader could tell you which plug it is. Used to be you would short out each plug wire from the distributor and the bad plug was the one that didn't change the misfire rhythm. Can't do that anymore.

 

I don't know what went wrong but a COP spark tester will test your coils/

http://www.amazon.com/SG-Tool-Aid-SGT23950-Tester/dp/B003YNHK3K

All the parts stores carry them.

 

Cylinder #3 misfire. I would inspect the wiring at #3 cylinder first with a visual inspection, kind of strange it started after you touched it.

 

Could swap a coil to see if the mis fire follows.

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