Shifting cassette gears from more teeth to less teeth hard and fast can cause some hard sounds. After reading this post and my comment I did a zwift games race/ride today, C level, and paid attention to my 'down' shifts. When shifting under load I could make a clunking sound...I'm using Di2 12 speed 105 on my Aethos...It didn't happen all the time and I generally could not cause the sound at will but it did happen...oh I'm using an 11-30 cassette so the gears have small changes in the number of teeth I think compared to the 11-34...I also use 165mm crank arms and a 50/34 compact chain rings...I'm only 5'4"/140lbs so it works great for me.
If anyone...whether mechanical or electronic shifting...starts to experience changes in shifting...not crisp/smooth, etc. check and inspect before making any changes or worrying to much...often it is something simple requiring a fairly simple and easy remedy.
A visual inspection is a good way to start...look at the position of the rear der idler and jockey pulleys...they should be in relative alignment and parallel with the cassette gears. Check the chain for a stiff link...look at the der pulleys for debris, etc. Check the cassette/hub body for smooth, straight rotation.
Check the rear der. hanger...even a minor bump, etc. can bend or move one enough to cause a problem...everyone who works on their own bike should own a derailleur hanger alignment tool. Yes you do have to remove the rear der but that is easy and the adjustment doesn't change.
Check for correct placement of the cable housing/ferrules making sure none have somehow moved...only on mechanical systems of course.
Run the gears while on a stand to watch and especially listen as you shift up and down the system. There should be sound but it should be smooth and consistent.
Test ride the bike to try to duplicate the problem, sound, etc. Not a bike ride but a test ride only to try to discover what is going on. Sometimes it works other times not so much.
I love the Etube app and find it more useful and fun than the shifter method for adjusting Di2 gears.
For mechanical systems I always adjust the rear der, using the barrel adjuster, to move the der up to one easier gear. As I'm increasing cable tension I'm listening for the sound of the chain just starting to make contact with the next gear then I stop and back the adjuster off until there is no sound. Generally it is correctly adjusted. I use the second gear...the 12 where the 11 is the biggest gear...to adjust a mechanical rear der.
For electronic systems I use the fifth gear down as is indicated by Shimano and pretty much every video out there I think.
This may be boring to the many of you that have been doing this stuff forever...I've worked in several bike shops, and still do, since the 80's...but to newer riders and especially those changing to newer systems it may be helpful. I had a bit to learn when I changed from my 20+ year old Campy 9 speed Carbon Record system to Ultegra 11 speed mechanical...I hadn't used a new/different system on my bike in over 20 years and while I worked on lots of bikes at the shop I had been out of the loop for a few years due to my primary job requirements...12 hour swing shifts, 6 days a week really really sucks...so there was a learning curve to catch up on and lots of videos to watch lol.
Hope some of you found this useful...to those that didn't...pfffffffft lol