It sounds to me like it could be something on the bike rattling.
The vibrations at that particular rpm range are causing something to make the noise.
It could be a loose bolt with the washer jingling. It doesn't sound like an engine noise from the description.

If you put the bike on the center stand and run it up to the rattle RPM, will it still make the noise?

If so, maybe you can pin-point the noise.  You may want to get a cheap mechanics stethescope (I know that's spelled wrong but I'm lazy tonight), and hunt for the noise.  I've had one for 20 years, and I've use the heck out of it.

Are you sure it's not your gear?  i.e. helmet, boots, etc...?

By-the-way, I used to race a "Tony Kart" - go kart.  Is that where the name is from?  What part of the world do you live in?

Paul

Pkaaso,
I am from Milwaukee. Yes I do race karts.  I have a 125cc shifter kart.  Did you race 100cc clutch kart?  I also race an Aprilia RS 250 regionally here in the Midwest.  Thanks for your replies.  This noise is driving me nuts.  I am going to try putting it up on the center stand tonight after work and run it through rev range in neutral.  I think it coming from the engine and I though possibly it was a loose valve.  The problem with that thinking is that loose vales usually get louder as rpm increases I haven't real dug into it but I thought I would ask here first to save myself some time..

Or, your keys rattling on the handlebar? Happened to me...

Yea, way cool! I raced a 100cc Yamahaa on an Emick kart.  Several years ago though.  We had a 4 track circut the we would race almost every weekend hear in So. Cal.  Wow, I miss it.

Careful not to over rev the motor in nutrual.

Keep us posted.
Pal

I have the same problem on my 2001, i cant figure out where it is coming from, i have a shade of 5k miles on it and it sounds like it is coming from the front of the bike.  Let me know if you find anything.