Humming/Vibrating Noise When Accelerating

I’m getting a loud humming/vibrating/knocking noise – like someone knocking on a door, only less loud – when accelerating and when I stop accelerating, the noise stops, but the actual vibration with a slight knocking noise doesn’t seem to end until the car comes to a complete stop.

So if I let the car roll without accelerating, I’ll still hear some of the humming/vibrating/knocking, but it’s a lot less.

Details about when the noise occurs:

• All types of acceleration

• Steering wheel doesn’t affect it.

• I can hear the noise, but I cannot feel the vibration in the car. If I turn on the radio on normal sound, the noise is barely noiser than the radio.

• The hum occurs as long as the wheels are moving, almost as if something is rubbing up against the wheels. I don’t see anything doing that though.

• No check engine light or any other warning light. I don’t know how trustworthy these things are anyway.

I just want to narrow what this could be, as I’ve had a mechanic deceive me a few times about problems and I had to return again and again, only to later learn the guy was being dishonest.

Car is a Kia Rio.

Update

Mechanic diagnosed the wrong issue because the sound didn’t go away, but not before charging!

The sound only occurs when the car moves and while acceleration seems to make it hum/knock more, even if I let it roll, it still hums/knocks some.

Update 2

The mechanic claimed it’s a transmission issue. The check engine light did come on a few days later during what felt like a gear switch (this is an automatic). I am leaning toward believing this, though I am a little confused as to why the car makes a knocking/hummning sound when the car is rolling without any acceleration? I didn’t know a transmission issue would do that.