Shimano 105 R7000 drivetrain makes crackling sound I got an Shimano R7000 groupset and have the following problem:

After degreasing my chainrings (54 – 36) and the cassette (28 – 11), the drivetrain makes normal noise. However after around 30km, the rear cassette (or chain?) begins to make noise in the lower gears. It is very noticable in the 36 - 28 (the smallest gear), 36 - 25 (2nd smallest gear), and 36 - 23 (3rd smallest gear) combination. Interestingly, the 36 - 21 or any higher combination does not produce the crackling sound. 

I made a video which illustrates the behavior. I‘m starting in the 36 – 21 combination which makes normal noise. When shifting in a lower combination, you can hear the crackling sound. At the end, I shift back in the 36 – 21 combination which is totally fine.

Questions:


Does anybody experience a similar behavior when the drivetrain is dirty?
Is there anything to fix this crackling sound?




Additional information 2019-07-09:


Indexing: I don‘t think the indexing causes the crackling sounds, because i experimented with the barrel adjuster and it had no effect on the crackling sounds - it only effected my shifting
L-limit screw: Is set correctly (see photo below)
Derailleur cage: In my eyes, it seems a little bit bent. It is the normal position of a Shimano cage? (See photo below)






Additional information 2019-07-30:


I changed the chain, no effect. Can the spokes produce such a pinging sound? If yes, how can I check spoke tension?
It sounds like your derailleur indexing is bad. The 'crackle' is the chain trying to climb the shift ramps on the next largest sprocket then falling off.

It may be that your derailleur indexing is badly adjusted. You can run through a rear derailleur adjustment process and see if this fixes the problem.

You should also check that your derailleur is aligned properly. The derailleur hanger can get bent so that the derailleur cage is not longer parallel to the rear wheel. If you pick rear wheel of the bike up and look down the line of the chain you can see if the cage is grossly out of alignment if it is not parallel to the chainrings. A bike repair shop can check and re-align the hanger. 

Another possibility is that the chain is catching in one of the jockey wheels in the cage. If you hold the rear wheel off the ground and pedal manually you should be able to see this easily.