Resonant humming noise at 118km/h

Question

I have 4 years old VW Jetta and when I am on highway going exactly 118km/h (73mph) I hear low pitched humming noise. It is specific to this exact speed (hence why I suspect it's resonant in origin), it get quieter when my speed varies by 1-2 km/h and completely disappears when the difference is bigger (can't hear it at all at 120km/h). It is loud enough to be clearly noticeable and cause annoyance.

I haven't been able to determine where is it coming from because I always drive alone. I hear it coming from right side, but since I am sitting on the left side of the car it might just as easily be coming from the middle. I wasn't able to determine front/rear position of the source either.

Regular maintenance service is coming up soon and I would like to bring this up. People in this country are usually not very competent when it comes to, well, pretty much anything, so I would like to know before-hand what might be the cause so I can point them in right direction.

UPDATE: I did some tests and the noise is tied to the speed of the car only. I tried to change gears, driving in neutral, tried both D and S modes and the noise still appears only at 118km/h and every time I hit that speed, no matter if I am accelerating, decelerating or moving at constant speed. Turning the AC and other electronics on and off did not have any impact on the noise

UPDATE2: I updated tire pressure to recommended setting and the noise is still there, unchanged

Answer 1

Turns out it was caused by one of the tires. All of them were pretty old but one was damaged a bit. I got new tires and the car now runs smooth and quiet. Thanks for all the help

Answer 2

If it's hard for you to diagnose it, it's nigh-on impossible for us to tell you what it could be.

That said, here are a few things to help you narrow down where the problem is so you can tally it with any findings your service technicians unearth:

• At resonant speed, put the vehicle in neutral and see if the resonance goes away

  If it vanishes, you know the issue is not related to parts or components downstream of the gearbox (drivetrain).

  If it persists, the issue is either related to the drivetrain, or not at all related to the engine or drivetrain.

• If you can change gears, try changing gears at resonant speed to see if the sound stays or vanishes

  This will determine for you whether the source of excitation is engine-related or related to the speed of the vehicle (in which case the motion of the vehicle is the source of excitation)

  Another way to do this would be to turn the A/C on or off to see if resonance is affected. No change = body-related vibrations, change = engine/drivetrain-related vibrations.