I’m trying to help a mate out here (and solve an apparently baffling puzzle). Apologies for the long post but the devil may be in the detail.
After my positive experience, he has fairly recently taken delivery of a new Mason with Di2. He loves the bike but noted quite soon after it arrived that the Di2 was noisier than he expected. It has worsened in the last couple of weeks.
He asked me to help, so I went round to his last night for a beer and to have a look (as almost last chance in the UK pre lockdown no.2). Over the course of an hour, I reindexed the gears, checked high and low limits, adjusted the B screw, and checked for wear; everything seemed in order. It actually shifts perfectly.
But the drivetrain is certainly noisier than I’d expect and noisier than my own Di2. The noise seems to be coming from the jockey wheel area of the rd, and improves when you manually gently pull on the rd to reduce tension.
It is worst in the 50/11 or 50/12 combinations, and the noise is almost absent in the 34/30, and better in the little ring in general.
The only other thing that is possibly relevant I can think of is that he has been regularly riding the bike on his trainer (direct drive) and the noise is (according to him) worst on there. As it was dark when I went round, I haven’t been able to ride the bike outside, just worked on it on the trainer and in the stand.
The only clue google has come up with is a thread here https://road.cc/content/forum/are-you-better-my-local-bike-shop-278381
where someone appeared to have a similar issue with 105 which they fixed by putting a mini-spacer behind the cassette.
The best local LBS IMO is booked up for 3 weeks, and another good one has just announced that during lockdown they are prioritising maintenance and repair on bikes bought from them. Mason have said they are happy to have a look themselves, but it’s a 2 hour drive, which he will struggle to do without taking a day’s holiday. There are obviously other places to try, but I’m irritated I can’t fix it myself (especially after persuading him it’s worth the extra not just for the shifting but as it’s so low maintenance ).
Grateful for any advice/a last thing or 2 to try before admitting defeat and calling in the pros.