If it's very fine it will be the traction wheels where it may be one is not turning. Try to free it up, as sending the printer back s a lot of trouble if you find which one is causing this.
It' very fine indeed, but it does not seem to be aligned with a wheel.
I have been in contact with HP support, they've asked me to email them a sample of the problem, so I had to make a photo of a photo to be able to email it
I attached it to this thread. Resolution is not extra, but I had to make it small for emailing it.
More on this to follow ...