Noise between Seat Tube and Top Tube

I suspect that your seatpost is graunching inside the frame as the rider's weight moves about while pedalling.

By inserting the post much further than normal, it will apply a different less-worn part of the post to the clamping area of the bike. But you've tried this and the noise continues.

So the next cause is that the frame is slogged out already in the clamping area, because of movement of the seatpost and the leverage afforded by the long unsupported length. You could do the seatpost clamp up tighter, but this is likely to be a short term fix.

Try unclamping the seat post, removing, and then do up the clamp without adjusting it. Then measure the hole with vernier calipers in multiple directions. You want to see the same number in all diagonals. Also feel inside with a finger - it should be flat in-line with the seat tube of the frame, with no detectable curve.

If there's physical damage, you should talk warranty with the seller as soon as possible.

Personally the super-long unsupported seatpost is a recipe for disaster IMO. I've cracked three frames in decades, and bent multiple seatposts by having them too long and/or too little inside the frame.

Lucky you! A creaking left crank arm is far more common than a creaking seat post, and a lot easier to fix. A mechanic will not be able to see this problem in the stand because it usually only appears under some load. Pushing the crank arm sideways (away/towards the bike) sometimes makes the noise, but not always.

Confirm for yourself by pedaling uphill only with the right side and seeing if it creaks, then only with the left. The loose side will usually creak more than the other.

It can be fixed by tightening the crank bolt to its specified torque, which can vary from 240-521 inch-pounds. The crank arm should be replaced if it's obviously worn, i.e. rounded on the inside instead of square.