Chain cassette noise

The most likely scenario is that the freehub/cassette on your trainer do not line up exactly the same as they do on your normal wheel, and you need to make small tweak to your indexing.

In your second video, it definitely sounds like it is trying to change gear.

this exact thing happened with me and not only on trainer but on road when riding on higher gears. tried a lot of thing by my self and found out that sometimes chain links and bearing runs out of lube or lube doesn't get to that inner part again sometimes. so you could try this solution.

I had a similar issue. I have a groupset Shimano 105 R7000 which worked very well with a SRAM chain (PC1130). When I changed the cassette for an Ultegra R8000 it made this noise. After trying a lot of things (lubbing and another SRAM chain) it turned out the SRAM chain was not compatible (or not so compatible) with the Ultegra R8000 cassette. Installing a Shimano Ultegra chain made any noise go away.

I spent some time making sure my indexation was correct but it didn't remove the sound.

However after about 100km to 200km the sound was completely gone.

My though is that manufacturers optimize the overall lifespan of the whole drivetrain by having a slighly too big a cassette while the chain is still young so it can fit well and last longer when its elongated due to wear and tear. And we would only notice this now because we are in an quiet environment with a home trainer. I will put an update here as soon as I change my chain to a new one to comfirm.