It's a '91 GS, with 83000Km on it (52000Mi).  

Strange sound @ 3000RPM.  
Place a sobwoofer near your glasses and dishes cupboard and make it work. You will hear a sound from the dishes.

I can hear a sound like this inside the engine.  

I'm quite worried. Oil consumption has not changed in 20.000miles. She drinks 1 litre per 1000Km. It runs fine, has good power on all RPM. Both cylinders work fine. Fuel consumption is average. It does not stall. Idles perfectly. Starts perfectly. But only this sound.

I just changed (4000Km ago, 5 months ago), to synthetic oil (AGIP TEC 4T 15W50) that dropped oil consumption from 1l/1000Km to 0.5l/1000Km. A mechanic told me that using synthetic oil could clean carbon deposits and worn out parts are now more noticeable.
Before the sound appeared, I had a happy afternoon of riding, about 500Km (300Mi) ride.

Should I suspect about cam drive chain? I never replaced it. What's the average lifespan for the cam chain? And about the chain guide?
Should I suspect about piston / rings wearing out?
Should I suspect a bent rod?

I've driven like this for about 100Km now.

Any ideas or advice will be welcome.

I once had a really loud vibrating type noise around the 4k RPM that appeared out of the blue.

It turned out to be the tank vibrating against the frame because the rubber bumper had shifted out of position.

When it happened to me it was loud and I was nearly sure the sound was coming from my engine.

Not sure if this is your problem but it is worth a shot.

I put full synthetic in an early eighties Z250c (single) a week later cam chain rattled like a Buddha Loves You,very easy to change on one of those though.I personally wouldn`t recomend it in an old bike.Coincidently my GS5 has just passed 50000miles still goin strong and pulling sidecar.(i use Rock semi in mine)

Is that sound at 3000 in all gears?

How's your chain's tension?

Michael

52K?...... GS5 engines have died a natural death with less than this on the clock.

Get the oil pressure on the main gallery checked and go from there, the procedure is described in the manuals   

Quote from: mister on September 10, 2010, 12:47:50 AM

How's your chain's tension?

Michael

I'm ashamed 

I knew my chain was not tight enough, but did not expect that this was the cause of the noise.

I'm using a 17teeth sprocket, and because it is bigger and the chain was not tight enough, the chain was hitting the lower cover screw, making a noise that did not sound like the chain, but stopped when corrected the chain tension.

I'm ashamed 

Thanks. I was expecting something worse. My GS is getting beyond the 50K line of the death... hope it'll last to 60K

Quote from: marc on September 14, 2010, 08:22:04 AM
..................I was expecting something worse. My GS is getting beyond the 50K line of the death... hope it'll last to 60K

Huh?  Now you tell me.  I put 80k miles on my 97 GS before I got down to a minimum thickness exhaust valve shim.   Have over 82k miles on my 02 GS now and expect it to go over 100k on my current mid size valve shims.     All depends on what milage you figure that adding oil is costing more than a new engine would, I guess.   

Quote from: gsJack on September 15, 2010, 05:51:34 AM
I put 80k miles on my 97 GS before I got down to a minimum thickness exhaust valve shim.   Have over 82k miles on my 02 GS now and expect it to go over 100k on my current mid size valve shims.   

That's a lot of miles!!!!

82Kmiles in 8 years is about 10.000 per year!