Hi,
there are several reasons why connecting external audio interfaces and musical instruments (especially guitars) can cause noise.
After you made sure all cables are indeed plugged in correctly and fully, in your case the most likely reason is that your guitar picks up noise from an electrical device through the pickup. If you turn down the volume using the volume knob on the guitar, and the noise goes away, that's your problem.
Sources for this kind of noise are lighting (fluorescent lamps and halogen lamps are especially noisy), but also your screen, or chargers or other power supplies may be the culprit.
Single coils are more prone to this than humbuckers. You need to identify the source of the noise, and remedy that, sometimes it's enough to turn away from the screen when recording, or simply off the a light.
If that is not the problem, check the cable from the guitar to the iRig, the shielding may be broken, try another cable.
The next thing you can try is make sure that it's not related to any power supplies that introduce noise. Disconnect everything from your iMac that is unrelated to recording guitar, all other usb devices, hubs, speakers etc. Connect the iMac only to power supply, keyboard, mouse, headphones and iRig, but not the guitar yet.
If the noise persists, try another USB-C cable, and then another USB-C port. If you still have noise, I would assume the culprit is the iRig. In that case, connect their support.
Then connect guitar, and then everything else, until the noise shows up again, and you have found the cause. What you can try then is:
- ensure everything connected to your iMac is connected to the same wall outlet (using a power strip)
- try another PSU (charger) and cable for the device that causes the noise
- ensure that the USB devices connected to your iMac do not draw more power than the port can provide (e.g. replace an unpowered hub with a powered one)
Let me know how that goes.
