Should I be hearing this?

[Feasty]

Had my '98 BMW F650 for a couple of weeks now. It's been running brill, not missing a beat. But recently I've noticed when I'm in gear and letting the bike slow itself down without braking, I'm hearing a kind of popping/over-run kind of sound.
It's not a backfire, as it isn't loud enough for that, and it's gradually getting worse. Any ideas what it could be?

I've left it a little while thinking it might just be this type of engine, but with it slowly getting worse I'm not so sure it's supposed to be doing that anymore...
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Previous: Aprilia Habana Retro 50cc (beauty), Yamaha SR125 (fell apart), Honda XR125 (nippy little commuter), Honda SLR650 (Geewhizz), Yamaha Diversion 900S (Smoooooth) written off courtesy of a stupid escaped horse.
(7 year gap), BMW F650 (Relaxing ride). Aprilia Caponord ETV1000 (Big and bold). Yamaha FZS600 (got me in trouble too quick!).
Current: Yamaha TDM 900 (Comfy, light but big, power when needed).

[Islander]

If it's getting worse then check the exhaust system for leaks - air may be getting in somewhere.

[P.]

Probably a tiny hole making its way into a bigger hole in the exhaust.

[Feasty]

Ah that's annoying if something has cropped up already! Not a surprise on this age of bike I suppose... (20 yrs).
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Previous: Aprilia Habana Retro 50cc (beauty), Yamaha SR125 (fell apart), Honda XR125 (nippy little commuter), Honda SLR650 (Geewhizz), Yamaha Diversion 900S (Smoooooth) written off courtesy of a stupid escaped horse.
(7 year gap), BMW F650 (Relaxing ride). Aprilia Caponord ETV1000 (Big and bold). Yamaha FZS600 (got me in trouble too quick!).
Current: Yamaha TDM 900 (Comfy, light but big, power when needed).

[P.]

I'm surprised my 7 year old bike hasn't started rotting, 20 years is fucking solid