WHY IS THERE A HUMMING SOUND COMING FROM THE AIR FILTER BOX WHEN I LIFT UP THE UPPER LID WHILST ON IDLE, WHEN I REV THE CAR THE HUM GOES LOUDER.

Your manifold gasket is leaking. Bad for the air fuel mixture and the ECU will try to compensate off the chart to keep the engine running.
Replace all the manifold gaskets with new gaskets or re seal the old gaskets with RTV gasket maker/sealer. (Do not use silicone sealant.) Don't over tighen bolts, ensure you tighten evenly across the flanges. Gentle with a plastic manifold.
That should sort your problem.
Good luck ?

I can relate my experience with a similar noise on my 1994 car with Traction control. A electric valve in the air intake made a "Hum" or "Buzzing" noise when malfunctioning. For my car, the valve was suppose to reduce engine RPM so the wheel spin was stopped by choking off the air to the engine.
So the electric valve on your engine may be getting a bad signal from the Traction control system especially since you are not applying power. Or simply the electric contacts in the part you mention are worn out and "buzzing" because the contact points are burned out and can not connect.

Follow these steps to replace the Air Filter on your Toyota:
1) Turn off the car before changing the air filter. Don't start the car once the air filter has been removed,
2) Locate the box that houses the air filter. Look in the front driver's side, back corner under the hood for a rectangular-shaped black box.
3) Lift the lid and compress the accordion-type tubing towards the engine. Remove the old filter from the housing unit. Clean any leaves or debris out of the box.
4) Replace the old filter with a new filter and pull the lid back over to the air filter housing unit, and snap it in place. Reattach the clips to secure the lid. Hope this helps.

most likely if its a hum it could be tires , if its a roar it could be the drive train

support the vehicle on jack stands , start the engine , put in drive and let it idle, if noise is gone ...its the tires if not check the drive train ...start with fluid levels

Hello,
I think the loud hum is coming from the exhurst. The exhurst either has a cut/hole or the rubber holding the exhurst has removed. This will make the exhurst begin to shake and vibrate.
Turn on the car, go under the car, then ask someone to press the throttle. This will reveal where the vibration is coming from.
Keep me posted.
Good luck.

too little information to go by..humming noise..what year/model/engine etc....

however i assume it s a ford and it is most likely caused by a sticky idle air control valve...they tend to hum and "hoot" when sticking, this may be associated with stalling at idle and/or hard starts unless aplying gas pedal. you can identify the idle air control valve as it is attached to most of the engnies by 2 botls and has about a 3-4" silver tube extending abaw from bolts, it is on top of the engine. when these make noise and backfeed through the air box a vibration can be felt from it. there will be a wire goinf to the valve, unplug it whil engine is running and when you hear the humming..if noise gone once it is unplugged...replce it WITH A FACTORY PART!!!

hope this helps

I suspect that one of your wheel bearing have worn out. SO to confirm please lift up one by one all the wheel and rotate. listen to any hum which corresponds to your noise. hold the wheel on either sides and check for any shake even a small one. if you detect a hum check the wheel bearings and the HUB, most cars have this assembly making the noise specially when the grease becomes dry or there had been water logging specially salty.If so change the bearings / hubs, when changing look out for any damage to the axle. I hope you get this right. Helpful? Good day.

MOST of the hums after a while come from the diff as the back axle does the driving and almost all its life runs the same way except when going in reverse ,is the rear diff meshing and worn on both the crownwheel and the pinion and the humming gets slightly louder the faster you go and almost goes when almost stopped and this can also be the slight vibration you get so get the diff oil drained and refilled and check if there is any metal in the oil when drained if so thats your problem But it will last for years like that..Let me know how you get on RON

Hello! If the humming noise is speed related(gets louder as vehicle speed increases)check front wheel bearings. If the noise goes away when turning one way or the other then it will most likely be in the front wheel bearing. If you turn the wheel(while moving)to the right and the noise goes away it is on the right side. If you turn the wheel to the left and the noise goes away its on the left side. Hope this information helps!

I am sure this is a worn wheel bearing, the giveaway is the sound change when you turn.