Whistling sound from the engine (with sound clip)

[BG DUKE]

Hi all, last week I was commuting to work with my bike and after 15 minute ride I noticed whistling sound coming from the engine. The sound appears in the moment after revving and you can hear it here:
http://soundcloud.com/nikolov666/ducati-monster-sound-clip-01-1
The sound appears when the engine is warm and after a few minute ride.
Last week I changed the oil from 20W50 to 10W50 both fully synthetics, but I'm not sure that the new oil is causing it.
It seems to me, that the sound is coming from the vertical cylinder head.
Few weeks before I tightened the rear belt a little, because it was too loose. I used the "allen wrench method" and the rear belt is more loosen than the front one. But no problems with the sound after the tightening.
I forgot to mention that the bike is 1998 Monster 600.
Any help will be highly appreciated.  

Thanks!

[stopintime]

Did you check that the belt rollers are rolling freely, no binding?

[Howie]

Did you check that the belt rollers are rolling freely, no binding?

And back off on the belt adjustment a little.  Does the noise go away?

Year and model?

[Jarvicious]

I'd say it's either your turbo spooling up a touch early or what the other guys said.  If I jack around with something on my bike and something wonky happens, I just backtrack and it sounds mechanical/bearing-ish.

[Speeddog]

Did you check that the belt rollers are rolling freely, no binding?

And back off on the belt adjustment a little.  Does the noise go away?

Year and model?

+1

[BG DUKE]

Thaks for the advices  
The belt rollers are in perfect condition and they are rotating very smooth with no binding.
Tomorrow I'll have enough time to check the rear belt tension and after that I'll tell you the results.
I hope I didn't break down somthing
Anybody else riding with the same oil grade 10W50 ?

[mattfwalters]

Weird. My wife's 2011 696, and my 2010 both make that same fluttering whirr sound as the engine decelerates. Hers has since 0km when we bought it, and mine has since 2,200km when we bought it. Are they over-adjusting the cam belt tensioner from the factory on the 696, or..?

-Matt

[Desmo Demon]

I noticed whistling sound coming from the engine.

You may want to double-check your crankcase breather while you are at it. The breather tube came off my 1987 Paso and it made all sorts of whistling and metallic sounds.

[zooom]

I think it is Bubb Rubb's patented new sound for Ducati's....

[Heath]

Thaks for the advices 
The belt rollers are in perfect condition and they are rotating very smooth with no binding.
Tomorrow I'll have enough time to check the rear belt tension and after that I'll tell you the results.
I hope I didn't break down somthing
Anybody else riding with the same oil grade 10W50 ?

Most people are running 10W40, next would be 20W50.  I don't think an oil change would be why you have a sound change.  No oil change has ever made my bike sound different.  Only feel or perform different.

[Duc Buz]

I think it is Bubb Rubb's patented new sound for Ducati's....

I saw him on some TV show the other day along with his woman and her fabulous grill, WTF.  Is that really something going on in Cali?  Only a matter of time until it shows up in Houston.

[WhiteStripe]

I think it is Bubb Rubb's patented new sound for Ducati's....

the Ducati goes whooo whooo....