Ok this is a problem we have had on out e tandem it has a Bosch motor in front. This lets my wife peddle with me not peddling our it les me back pedal without her peddling.
Other than that the drive train is pretty normal only two chainrings both narrow/wide 10 speed. It has a 11-42 Shimano 10 speed cassette and a DT Swiss 540 Tandem/ ebike rear hub. This noise started sometime this winter. I thought it was my wife over or under peddling because I have been tired and not starting off as hard as I used to. But when I rode the bike by myself it still happened. So you get the wiring noise you hear in the video and sometimes the clunk at the end. It's happens when starting off or peddling really slow or when I let up or let up to shift. I can somewhat eliminate it by starting in a small cog so it takes a lot more force to start. But if we peddle slow enough then it will do it no matter what and the clunk is harder.
The weird thing is the drivetrain is smooth when this happens and you cant feel anything when it makes the wiring sound. But you feel the clunk but I think you feel it in the drivetrain but just in the bike. But it's hard to know for sure. We had it in the shop and the mechanic thought it was the front chainring was worn. It's only 18t because it's 2.5 to one gearing. I have no clue where the sound and the clunk are coming from. There is no slipping like we have with a worn cog or when I used the wrong grease on the drive. The drive feels normal and wit the vide I can see it is acting normal in back. My wife is blind so she could not see if it was working right.
Sorry about the camera angle I shut taped it on the frame.
I replaced the front chainring and the timing chain as it was time the back I just changed a few months ago and the cast only has maybe 200 miles on it.