So went to the dealer today, they took it to a test drive and said they couldn't hear anything
Typical. When I showed them the video I took, even then they said 'nope, can't hear anything'. Only thing they said was that by cleaning the chain, it might have a different sound than before because the links are more flexible. Also asked about the chain being maybe a bit loose (got about 5cm of slack which seems too much). Nope, they didn't even wanna take a look at it, it's fine.
Drove to my friend's house (who lives 3km from dealer and has the same Z900RS), he took it for a drive and immediately noticed the noise. Definitely a chain-noise. His chain has way less slack than mine too. Not going back to the dealer, they always make me feel like an idiot asking to check things, everything is always "ok" and it seems the main goal is to get me out of there asap (friend has same feeling). So asked a local dealer (other brands) if he would like to take a look at it, awaiting response...
Anyway, I suppose it's definitely not a mechanical problem since dealer won't take a risk letting me ride a bike within guarantee with a problem, might indeed be that cleaning the chain has made the noise characteristic a bit different, and this coupled with a possible too loose chain (dealer said cleaning might have caused the links to get a bit further apart than before) might be the cause of the buzzing sound when there's tension on the chain. Maybe the chain just rubs the swing-arm. Dunno, hope the local motorcycle dealer has more than 3mins time to take a good look at it.
Kinda disappointed, bought 2 X Z900RS together with my friend 2 months ago, very friendly during the sales talk but once delivered, they got no time. Really annoyed by this. Anyway, on topic... do you guys think a chain that's a bit too loose might cause a buzzing/rattling noise that wasn't there before?