Zoom G1 Four noisy output to amp or audio interface

I bought a Zoom G1 Four fx unit two weeks ago and I really like it. However it is quite noisy when using a mono jack cable to connect to a guitar amplifier (Yamaha THR10 and Blues Junior), or my recording interface (Focusrite Scarlett). It is a hissing / sizzling / popping kind of noise that gets louder as I turn up the amplifier gain / volume or audio interface input gain.

However, when I use stereo headphones in the output instead it is completely quiet with no evidence of the noise at all.

The noise isn’t generated by a patch as the noise is still there when a blank patch is used. I have tried a variety of different guitar cables and the noise is always there. The noise is also there with nothing plugged into the input. Thinking I had a faulty unit the shop I bought it from replaced the unit with a new one. The second unit has exactly the same issue.

I queried the problem with the Zoom Corp website contact page a week ago and received an automated email stating a ticket has been raised but I have not had a response as yet.

I have searched the internet / YouTube for any reference to this issue but I have found nothing, which leads me to think the shop maybe has a faulty batch.

Has anyone else noticed this on their unit or can anyone confirm that the output to an amplifier or audio interface on their unit is silent?

Thanks in anticipation.

 

Following suggestions on another forum I have done some further testing and discovered the problem.
I have been powering it via usb from my pc and have it going into my Yamaha THR10 which is also connected via usb to my pc. With both usb cables connected I get the noise, but take one or other usb out and the noise is gone! I never considered that USB’s could be so noisy.

 

I have ended up using rechargeable batteries and it is silent with them, but your idea sounds like another good option, thanks.