Strange noise or Pulsing?

[Piercee100]

I use a Suzuki Inazuma GW250 for work. It's just an unloved commuter but feels great tearing from work and always seem to red-lining it home, which being a 250 means i'm not far over the speed limit.

Recently the powers that be have stuck average speed cameras on my route home which means holding a constant speed over two seperate 10 mile stretches. The first to appear was a 40mph run on a quiet country lane. So I sit in 6th gear at 40mph around 4k rpm and I hear this swish,swish,swish noise like my front brake is binding slightly. I brake but it doesn't go away. I pull the clutch in and the noise is gone. So something in the engine or clutch causes the swish, swish, swish noise which is quite annoying when its all you can hear.
This week they installed another 50mph average speed camera run on my route home and I notice that my swish,swish, swish noise has been joined by a slight pulsing visible on the tacho by a +/- 100rpm wiggle. Feels like a slightly buckled wheel or tyre. As soon as you throttle, decelerate or clutch in it's gone.

Gear or tacho speed don't seem to make a difference, it's just the holding of a constant speed that does it. On a boring run it's all you can focus on and its doing my head in. Any ideas what it could be?
Only really discovered this recently because I've never had to maintain a constant speed before, its usually throttle on or throttle off on my way to and from work as traffic is minimal.
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Rides: Honda H100a (Retired), VT500e (Sold), Kawasaki ZX400 (sold), Kawasaki GT550 (Sold), Suzuki Inazuma (Daily), Honda CD185t (Broken), LML Star 2t (Last run edition).

[Rogerborg]

Sounds like what my Nazi Tractor does. I've never been able to find the cause. I suspect it's just some weird harmonics. If one of our bikes explodes, we should update this thread.
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike