New chain, new cassette, new trainer...new noises I recently got a Tacx Flux 2 Smart trainer (because lockdown) and decided to get a new cassette and chain as well, since I won't be doing any riding anytime soon and would like to keep everything nice and clean in the house.
I installed the cassette on the trainer,checked I was using the right adapters for cassette and QR, and then measured the new chain against the old one (they are the same KMC 9 speed), making sure they were exactly the same length. I then installed the chain with the supplied quick link, making sure it is routed the right way through the jockey wheels.
However, I now get a very clear droning, grinding noise and perceivable vibration when in the smaller 3 rings. The noise almost disappears when in lower gears. The shifting fine though, and I can get shift in all gears.
I have no idea what can be wrong and it feels almost like the chain has a hard time slotting into the teeth of the chain rings. Almost like the each roller slightly hits every tooth before grabbing on. If I had to describe it in an onomatopea, it would be a grindy &quot;RRRrRrrrrrrRRrrrRrRrrrRr&quot;
What could be the cause of this? Is it just because the chain and cassette both need to be bedded in? Is it lubrication?
** UPDATE **
I finally managed to ride the bike outside and cannot reproduce the same noise or vibration. It seems that the issue is with the bike one the trainer.
I got the same with my new COVID-Kickr, in my case it was simply the alignment of the cassette/trainer with respect to the frame/rear-derailleur/chainline.

For whatever reason the usual stops in the frame that ensured same/similar alignment when mounting the wheel just wasn't quite right for the trainer (skewer QR system here).

It's hard to notice as without the wheel you can't check the placement with respect to the front of the chainstay (or brakes in terms of 'roll' misalignment) as you would with a wheel (assuming it's correctly dished).

If you're grinding in the bottom 3 then I'd suggest your first tweak would be rotating the trainer slightly counterclockwise (looking down) with respect to the frame, this amounts to relatively more rearward in the left dropout, and/or more forward on the drive-side. If it's better then that's a win, if it's worse then go the other way. 

Either way it's helpful to get someone else to ride while you investigate (wish it were so easy on an actual bike huh!)