Hello!
Recently I've noticed this strange noise. This happens at around 5k RPM and only when the bike is moving. If I just twist the throttle in neutral, it sounds fine.
Please have a look, I am talking about the 'humming' noise which starts and stops periodically at the first half of the audio:
The recording is from the phone in my pants, I am not sure if anyone can hear it outside 

Does it sound like something familiar to you? Should I worry about it?

Thanks!

First of all you should do a cleaning and maybe also a synch of the carbs. thats not steady at all. it runs fine?

exhaust headers and can could be loose also.

Hi, thanks for your reply.

Yes, it seems to run fine below 5k or once it passes 5k. But it's hard to tell for me, I've had the bike only for a few months.

Otherwise I had a similar behavior a few times at lower RPM, but I wasn't recording at that time. It sounded totally strange and scared me.

Finally I've been able to record this. Here I accelerate from 1st gear, switch to 2nd, and at around at 13-15 sec. it starts running unstable.
https://www.dropbox.com/s/bdk96xw23v61gn8/Engine%20noise%202.mp3?dl=0

It doesnt sound that bad in the recording.

I would isolate if it's vibration from something loose or without rubber dampers(tighten the headers, check the tank and other things that could be vibrating)

Or if it is from the engine sound , mainly irregular combustion due to carb issues.


As you say it is a scary sound, it could also be rattling from the cam chain if it is loose, and is very dangerous.

Checking the cam chain tensioner is not a bad idea, they can fail or get loose and is a disaster (torque the bolts and loctite)

This definitely looks like a potentially serious issue.

Tomorrow I will visit the mechanic, I will ask him to investigate that, I'll provide him the info you've pointed out. Later I'll update the thread.

Thanks again for your help! 

Hi,
You were right, the mechanic has cleaned and synchronized the carburetors, and the issue is gone now.