Have to ask....how confident are you that it is the seat post?
With the noise you describe I have found this to be either pedal related or the actual seat itself.

That is a fair question.
It was making a similar sound 2 years ago when the bike was new, and that time it definitely was the seatpost. I took it to the bike shop where I bought the bike, initially thinking it was bottom bracket (and so did they). They spent about four hours systematically substituting each component to find out where it was coming from, and ultimately found it was the seatpost. They greased the seatpost bolt and wedge components as I described, and the problem went away after that for almost two years. Now it's making a similar sound, and once again it's only when I'm sitting in the saddle, so I figured it was the same issue that had come back. However, since I'm not making any progress I think it makes sense to look at other possibilities. I think saddle is more likely than pedals, since it's only when sitting down, but it's easy enough to try different pedals. I will try switching pedals, and then switching seats, and see what happens. Thank-you for the suggestion.

could it be that your seatpost is at maximum insertion? meaning it is as low as it can go and hence touching the seat-stay junction where it can't go lower? in that case the contact would cause the creaking. if so, you cut your seatpost 5mm and you are good .

No, but interesting that you bring that up. I did come across an online manual which recommends that no more than 210 mm of the seatpost should be inside the seat tube. The reason is to leave a few centimetres of clearance so that the seatpost does not hit the frame if it slips, otherwise it could be dangerous and cause damage. It turns out my seatpost is lower than the recommended limit, though still not hitting the frame (I've checked and there is still a couple of cm more to go down before it hits). I don't think that is what is causing the sound, but I do intend to have the LBS swap it for the shorter version of the seatpost for the safety reasons mentioned. I am quite annoyed that the LBS did not pick this up since they are the ones who assembled the bike and did the fitting, and of course they did not give me the manual ("Just ask us..."). Again, I don't think it's related, but in case anyone's interested here's the link:

https://media.specialized.com/...teral/0000097485.pdf

That motherfucker is probably about to break. I had a very similar issue with my mountain bike (Specialized Epic). I couldn't figure out why there was a creaking sound and one day it just snapped while I was on a trail and actually damage the frame as well, which was a bitch to find someone to fix.

I have a buddy who sees this a lot. 1st he puts the grit/paste for carbon to carbon interfaces on the seat post and that solves 90% of the problems. 2nd change the chain. Between the 2 almost all resolve. He charges $50 to put the paste on the seat post because people will pay anything for the noise to leave. He usually gets a 12 pack to boot.

Ok, I’ve made some progress. I went to inspect the seat to see if it was loose or damaged, and I discovered that I was able to replicate the sound by wobbling the seatpost side to side. Even thought the bolt was tight the seatpost was still able to wiggle inside the seat tube. So what I did is I took the wedge out, and this time I took the seatpost out completely and put the wedge in first and the inserted the seatpost — before I had always tried to put the wedge in with the seatpost already in place and I don’t think it was fully inserted. This way seemed much more snug, and the first 15 minutes of my ride were gloriously silent, but then.... it started to come loose again. I think I’m getting close though. I will watch some YouTube videos on wedge insertion (yes, there are a few). If need be I’ll buy a torque wrench to see if I can go tighter on the bolt. I will also keep in mind the carbon paste option. Thanks all for helping me brainstorm.

Hi
If all else fails use Teflon paste the stuff from the hardware store for pipe fittings.
I have a new R5 with the D shaped seat post after a rain ride it would raise hell.
Thousands of miles nothing but Shitty E Tap shifts.
Grumpy Old Man

May well not be this, but with bike squeaks / clicks and random things it normally turns out to not be the thing you think it is.

I had creak from my seat post for a while. Went though all you had as well. After many many months I tracked it down. Where the right hand saddle rail went into the back of the (specialized) saddle at the back that had somehow started to creak. So only happened when seated, etc etc. After taking seat post out to grease (alu frame) then it was ok for a while, but in reality it was as I'd pulled the seat up to get it out.....

Solution was dripping some oil into the saddle / rail joint and years later it's been fine.

Get some carbon paste grease and use that ASAP, easy option to see if that is part of the issue, won't need to worry about a torque wrench then (but that is always a good option for your tool kit).

N of 1 here. Have a similar reoccurring knocking/creaking issue with my SL6 and my fix is to tear down and thoroughly clean the post, seat tube (using flexible brush to reach to BB), AND saddle mounting/adjusting hardware. Post and seat tube get fresh carbon paste, fresh oil on seat post wedge bolt and saddle mount bolt to deal with FL "perspiration". Had to complete this every 3-6 months initially but haven't had to do it in about a year. Good luck!

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"This ain't no wedding dance, get moving" - SS

So I did the carbon paste tonight and I *think* it solved the problem. I only went for a 40 minute ride after, but so far so good, so hopefully it stays that way. Thanks again to everyone for all the helpful advice and comments.

I can see you smiling from here! Well done. These noises can be a complete PITA!