Why is there a humming sound coming from the off side of the engine bay

This may sound crazy but that model astra would get fuel smells inside the car from a worn petrol cap, mine did, I spent ages checking the engine bay etc.
If it is not that, check the engine bay for injector supply tubing leaks, check the inlet manifold is not loose, and check under the vehicle near the rear drivers side(UK) for the fuel filter as they can corrode.

when did you last have a timing belt change . may be a pulley wheel bearing failing.

have you checked all of the idler pullys and the tensioner for the serpintine belt take the belt loose and spin the pullys also check the alternator with the belt off

Hi:
Often "humming" noises like what you are experiencing are caused by a failing bearing somewhere. Some usual suspects are wheel bearings, alternator bearings, and sometimes even your waterpump, but usually it starts leaking too. To start narrowing down what it is, have someone ride with you to see if the noise is coming from left side or right side, or engine area, then see if it happens when the car is not moving, or just when the drive wheels are pulling the car? This way , hopefully you can pin point the culprit!

humming or whistling noise..if it is a humming ..try this...going average speed turn your wheels side to side if the humming goes away and comes back then it is a wheel bearing

A car's steering wheel will vibrate when driving if the car's alignment needs to be adjusted. also possible bad engine or trans mount
have them inspected .
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Rather than guess witch one it is( sounds to me to be a wheel bearing or hub bearing) just chg. out both, but if it gets louder when you make a right turn i would say its the left side sense when you make a right turn you are putting more forse and weight on that side.

SOUNDS LIKE ONE OF YOUR WHEEL BEARINGS HAVE GONE WEST REPLACE THESE BEFORE YOU GO KOOFING FOR A DEEPER PROBLEM

TO CHECK TURBO = + WITH ENGINE OPERATING OPEN HOOD LONG SCRDRIVER(SAMPLE TOOL) TOUCH OBJECTS, PUT UP TO EAR, SAMPLE SEVERAL ITEM TO GET TOKNOW SOUNDS, BE CAREFUL NOT GET CLOTHING OR ,CAUGHT . TO CHECK WHEEL BEARINGS, JACK UP ONE SIDE, GET TIRE OFF GROUNG ENOUGH PUT HAND UNDER BOTTOM, THEN ONE ON TOP,TRY TO ROCK IT IN-OUT, IF NO PLAY, IDEA WISE THEY ARE OK.

check for leaks in the fuel line from the gas tank up to the engine bay.the humming sound you hear at the back is your wheel bearing.it needs to be replaced.