I use a PlantronicsDSP500 usb-headphone, which in general is very good and pleasant to work with.

But now I hear after a few seconds crackles that will end in a very noisy distortion of the sound. It sembles a static kind of noise.

What could be the reason for this ?

It appears in Final cut pro, but also in Peak.

Aside from the fact that it's USB? Given that it's Plantronics, it could be something to do with an electret element, or piezo element, or that your system is stumbling with resources in the process of recording the mic or converting it back to analog. (When you use a USB mic and monitor it, it's an analog to digital to analog conversion, and you're already starting out with what is likely a compromised signal)
Try defragging, first, would be my recommendation.

Does the crackling appear on the recording? If not then it is probably the USB driver for the headphones being unable to keep up with USB traffic (including the mic in) and headphones out. If it does appear on the recording as well it may still be the same thing but would be nice if the software protected the incoming path at the expense of monitoring.

In either case you may want to try putting the mic and headphones on different USB busses. Also get rid of any hubs you may have in the system.

A. J. deLange: I think u are right. I use a hub (not for the plantronics btw) and it occurrs when other jobs were running.

I also thought it could be caused by Coreaudio?? Because I always have trouble with peak and Coreaudio ('cannot find Coreaudio', it says when starting up).

Anyway, right now the Plantronics is working fine again.

Kind regards,

Peter Schmitz