[Antipodean Andrew]

What's the whizzy noise UCE RE make on deceleration? This is not the engine itself, but the gear train. I know it is normal because I have listened to many videos to see if my wonderful mechanical noises are to be expected. Whizzy noise experts... your knowledge please. My bike is very new, so this may mellow out in time, or not.

[AzCal Retred]

The Bat-Turbine spooling down??    My Pre-Unit makes enough clatter that I usually have earplugs in. Straight cut gears are famous for whiney, whizzy sounds. Maybe the primary chain shoe tensioner also, when unloaded the chain would rub over it a bit differently.

[Antipodean Andrew]

Yes, Bat Turbine spooling down ... that's it.

[Antipodean Andrew]

Charcoal canister my arse.. it's a turbine! Fancy Royal Enfield turbo charging our bikes and not putting it in the advertising literature.

[axman88]

The final drive chain also acts follows quite a different path during deceleration, but this would be lower frequency clatter.

You might want to investigate primary chain over-tensioning.  Not saying that's where your sound is coming from, but this is a known issue with some UCE machines.  If the chain is over-tight it can lead to primary failure and more damage.

Recording and posting a video or audio recording is a lot more accessible and informative way to get help with mystery sounds, than attempting to describe sounds in text.  I bought a cheap Go=Pro knock off for this purpose.  Although the video and audio quality are not great, or even good, it is better than nothing, and came with a waterproof housing, and included a handlebar clamp that let me attach to my engine guard bar, so I could aim the camera at whatever I was interested in recording while riding.  https://www.ebay.com/itm/1080p-HD-Action-Camera-Wifi-Sports-Camcorder-Video-Recorder-Waterproof-DVR-Black/233610682665

These things come in an amazing variety.  Two different ones that I got my hands on, that looked identical, had differences in lens length, audio quality, and firmware.  Both include rechargeable Li battery and use a micro SD card to record on, which was not included.   The more recent ones have WiFi connectivity.