Hi everybody,
After your suggestions I tried writing something with 24 channels, routing 4 instances of EWSO Gold, one for each orchestra section, but there is an interference noise, even when I play a solo track or mute everything else. I can't understand the reason, can you please help me? 
I use Sonar 8.5 with Windows 7 and 4 GB RAM.
 
Thank you in advance.

What sound card & drivers are you using? CPU speed? 64 or 32 bits?

Check all your cables and connections. Every time I have mysterious noise it is in the cable connections somewhere or is a bad cable. 
John

Thank you for answering.
I don't think the trouble lies in the hardware, because the noise is rhythmical, it seems like a track with the wrong pitch and the wrong instrument.
My DAW is a bit dated (Core 2 T9300, 64 bit) but since it's a laptop, the only cables are the ones for the soundcard (a cheap Soundblaster, but it has never given me problems in the past).
 
Any suggestions? Thank you again.

4Gb of RAM in a 64 bit system? Might be a bit on the low side

Rather than mute or solo, does the noise persist if you archive all but one instance of your synth?

Thank you for answering,
meanwhile by trial and error I found the solution:
I raised the latency up to 97 in the EWSO Options, and the now noise is gone

PS  With one instance there was no noise.