I have a PRS 513 and a Custom 22...
2 beautiful guitar!

I noticed that the 513 is noisy (even in the full humbucker mode, using the bridge humbucker or the neck one) compared to the Custom 22 (and compared to any other guitar with "normal" humbuckers). This kind of "buzz" is not so loud and it does not compromise the sound of the guitar (I used my 513 on my gigs in last 5 months with no problem, I noticed this noise only in comparison with another guitar with humbucker).

Is it a 513's pickups normal thing or is there something wrong in my 513?

Any info about the specification of the 513 single coils? I think that if any single coils is slightly different from the others (number of the winding) the noise I hear could be due to the slight differences of the coils in humbucker mode (as in the Gibson Burstbuckers).

Could you help me?

sorry for my bad use of english language, I'm from Italy and I'm not so clever in writing english.
I checked for any ground hum issue but I find no solution... The noise is still audible even if I touch strings or any other metal hardware of the guitar.