Perhaps check chain rub when it is under tension. Just spinning the cranks backwards or in your lightest gear probably won’t reveal anything.
Think I have something similar on a Volt (but, according to Wahoo, that is a Kickr copy…). For me I think what happens is that the frame flexes, and it makes the chain rub on the front mech cage at a point of the pedal stroke (in the small ring, it rubs on the chain catcher). Outdoors there is no such frame flex, because the bike just leans slightly, but the fixed rear axle makes it flex.
It is probably solvable with really good front mech alignment, but I’m rubbish at that. It annoyed me enough to puy my 1x MTB on the trainer instead!
It’s probably not this - but check the chain is routed thorough the guide between the rear derailleur pully wheels correctly. I switched from Shimano to SRAM recently and managed somehow to screw this up! Took me a while to realise the chain was incorrectly on top of the guide.
Curious if anyone’s had any luck with this or just rode through it on zwift or otherwise. Just swapped free-hubs, and the grinding was insane with a brand new Sram Rival cassette / old chain so I took the old cassette off the wheelset I was using and threw it on the trainer. Helped a bit but there is still some grinding in lower gears. Will do a ride this evening on it to see if it gets any better. But uhh… needless to say, pretty frustrated.