Why am I seeing subtle horizontal lines? (Intel HD Graphics 4000)
As a general guide, if it doesn't show up in a screen-shot, it's a hardware fault.

Potential fail-points are the monitor's video cable, the screen, or the graphics chip itself.
Unless you fancy stripping it apart yourself to try to re-seat or replace the video cable, I think a trip to the repair shop is on the cards.

iFixit do some good teardown &amp; repair guides, if you want to tackle something like this yourself - MacBook Pro Repair - though you'd need to see if they have one for your specific model.