I would leave it alone and just remove the belt.

 

I went to look at my belts on my lunch break (at work, can't wrench on it right now) but my belts seem a little on the loose side, though I can't imagine a loose belt making ^ type of noise. Regardless, I will try to to remove the belts, test everything spins freely after work today. I forgot to mention I did a pulley delete a few months ago (late-06 G/Z), so no more worthless fan-clutch pulley.

 

Yesterday after work I removed the belts, it appears everything on the front of the motor is fine. Everything else spins free, although the power steering pump has some end-play, but still spins smoothlty. The sound seems like its outside the motor, so I had a look under the car. I took a soft-blow hammer and tapped around the exhaust, and when I tapped the passenger side catalytic, it made a similar noise. Im going to pull the Y-pipe off tomorrow night or this weekend and see if the internals are dislodged. I have a feeling they are, but I'd already bought a brand new starter from AutoZone on my way home from work yesterday, so Im going to compare it to the Oreilly's reman unit that is in the car while I have it up on ramps.

I used to have an old Accord and when the catalytic went out on it, it made loud bang noise when starting and revving the engine, so hopefully its the cat from a diagnostics sake. Maybe it will be an excuse to need to put HFC on... but damn do they look like a pain to replace.

 

Still have not been able to solve or even diagnose this problem. New autozone starter made no difference. I checked all the torque converter bolts while I was under the car too, all good. I changed out the bosch oil filter to an OEM nissan filter last night, thinking maybe the drainback valve was bad causing dry starts, but alas, no change. I'm going to spend more time looking at the exhaust system, hopefully this weekend. Been busy with work this past week and fixing other cars with more critical problems.

 

Could it. Be your fan clutch pulley going bad?
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Possible play with the pulley could be resulting in the clunky noise you hear when you first startup the car.

There's a video on YouTube I just saw the other day on this issue. Search 03 G35 fan clutch pulley replacement

 

I'm 04.5, so electric fans. I actually deleted the mechanical fan pulley all together, so shorter belt like late-06 and 07 Coupes.
I checked my battery voltage at cold morning, it was about 12.3-12.4 when I went to start up the car. Battery has a build date January 2016, so I'm going to pop by Oreillys and have it tested. I'm still concerned its either something still with the starter/flywheel or something exhaust related, as it really doesn't sound like anything to do with the accessories. My work has now officially been shut down 'indefinitely' due to Covid-19, but rumor is we may be back in 2 weeks. Bummer, though it should give me plenty of time to troubleshoot this car though! My type-A is already kicking in lol.