So my Astra J 2013 1.4 was making a chirping noise while idling, removing the oil dipstick stopped the sound. After replacing the rocker cover, still made no difference. The actual cause of it was the Crank Pulley Seal.
To get to this seal you need to support the engine, remove the left engine mount, take off the cambelt etc and behind all that you'll see a circle seal. Replacing that solved it instantly. It had a small leak causing air to get out.
The video above isn't my video, but is the exact same noise and had the same fix.
Hope this helps anyone who has this annoying noise.