In my 09'XT660Z @7000 km I started to note a metallic vibrating noise around 4500 rpm (started around 4200 rpm and goes after 4700 rpm). Its noticeble with same intensity and pitch in any gear, accelerating, deaccelerating or at constant engine speed, even with engine cut off. It's more easily heard while moving than stopped.

It seems just like a loose screw that vibrates but with a higher pitch more like some sort of a vibrating whistle.

The bike was full stock when I first notice this. I have excluded knocking since it is a stock bike with low milage, although as it is very lean from stock I have installed a variable resistance to the intake air temperature sensor to rich the AF ratio. No other mods than this and DNA ring on air box snorkel.

I noticie this with warm/hot and cold engine.

Most of the time the noise is there but sometimes it disapears for a few miles and then comes back. Now I'm starting to notice that it appears with early rpm than before.

Can anyone help ? or konws about a anything like this on this bike?

It does not sound like any major problem, it's just annoying....

Ahhh, no bars nor mettalic bash plate on the bike (just oem plastic one)