In X Series devices, an ISO internal system optimizer is available on the front panel menu of the DSP. The ISO function has an automatic gating function activated in idle state.
This function may cause an audible, unwanted audio effect or a ramp-up effect. To remove the audio effect, find the ISO parameter using the front panel menu of the processor. Then change the Bypass to On.
Press enter twice to confirm the settings. Reboot the processor and the effect should be gone.