as you are soldering, it looked like you added the capacitator legs directly to the lipo in pads.
it might not be the issue, but to get the best perfoemance out of the cap; cut the legs to about 1-2mm lenght, add a 20awg wire and solder it to the lipo on pad. that way it will work better and the cap can be secured well, as example with a shrink on a standoff.

So i've experimented a little more and nothing of the proposed solutions seemed to work.
I've also tried to drive the motors on different speeds using the betaflight motor testing feature, without encountering the noise.

Now, I've turned on noise reduction in the receiver tab with a factor of 150. With those settings the noise is **almost** gone, only occuring on very low speeds.
I'll try to fly it soon and see whether the quadcopter flies any better now.