Big THANK YOU to the various suggestions!

I sprayed silicone lubricant on the seatpost and tightened the quick-releases. I also put silicone lube on the front riser stub. Hey presto! No more squeaking! It makes a big psychological difference, the bike feels much better to ride without the chorus of squeaks and groans

(I wasn't very scientific, since I did all 3 at once to save time. Hence, I don't really know what was causing the noise. For my money, it might have been the quick-releases. The lower one is a little awkward to get to and was probably looser than it ought to have been. But hey, silicone lube was only $4 and now all my closet doors fly open too!!)