I hear that sound regularly on my engine too, it comes and goes. From what I was told its pretty normal.
Ride it until the bolts fall off!

 

it's normal even if it comes from only one of the cylinders? really?

 

I didn't think that your engine sounds too bad. Sportsters are noisy engines. I never noticed any odd noises in my engine but after switching to a half helm, I started hearing all kinds of weird stuff that my dealer says is normal.