Car will not start and when key is on making a humming noise. what is that?

Not always, just replace the cap to start with. Sometimes they howl an other bearing make a noise.

The humming noise might be your cooling fan(s) coming on with the key in the "on" position. Check your battery connections for corrosion. Have someone jump the battery to see if the additional amps will turn the starter. Sounds like a bad starter though. I would need to know the year, make, model to be more helpful.

That humming noise may be your fuel pump burning out. Try turning the key to the on position while someone else listens near the gas tank. The humming should only hum for a few seconds so you won't have time to listen yourself unless you can hear it from inside. If the hum continues and doesnt shut off, it is likely a bad fuel pump. It is normal to hear a small hum for a couple of seconds , but when it starts getting weak, it will hum louder, longer, or not at all

starter

check your battery terminal wire connector make sure they are secure and clean of any corrosion . and as for the humming sound I believe that is your fuel pump running when you turn your key to the on position

post a photo with finger pointing to object humming.
we can not ever guess this.
there are 3000 parts on the car, and ive seen like near each make noise moving fast, 1 time the other.
1: check all fuses
2: makes sure battery voltage is at 12.6vdc rested, key on still close to 12.6vdc , still makes hum.?
my guess is the battery is drained but no proof, im not there use my voltmeter.

im going to say an almost dead battery .the hum you hear might be the fuel pump.you never heard it before because the van normaly starts .so boost your van if it starts you may have to replace the altenator the guages, no worries many cars do that .thats a code needing erased and if you disconnect the battery that may go away.

power steering pump make humming noise and alternator with bad shaft bearing make noise.

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Your fuel pump is supposed to hum, when you turn the key it primes the system and it will hum. When the car is running the fuel pump is running and it will hum. If the noise is excessive then try a higher quality fuel pump, it may be less noisy. You will hear the fuel pump on a lot of vehicles tho when you are sitting at a stop light with your radio off and everything quiet in the car.