Fast Track + iMac?

Question

macOS

There is an iMac mc-309, an external m-audio fast track audio card, and two M-Audio BX5 monitors. A couple of days ago I connected connected this whole structure. The first two days it worked fine, but on the third day something strange started happening. Sometimes, instead of sounds (more precisely, together with sound), a loud short-term hiss and crackle is emitted from the speakers, exactly for 1-1.5 seconds. This is repeated several times in a short time, then once an hour: I did not find any systematicity in this. I reinstalled the driver, took the latest version from the site. On win8-win7 audio and speakers work fine without any extra noise.
This happens with absolutely any sound, be it a system alert, a game, or the sound of a new message in a contact or Skype.