Weird accelerator pedal viberation

I can't say for sure whats causing the sound, if the engine makes no weird noises when reving it at a stand still it is most probably not the engine causing the vibrations.
It's either the wheel balancing issue or maybe the engine goes bizzare under load.
Anyways, if the whole car doesn't vibrate and only the throttle peddle does: It can be something tugging at the throttle cable or maybe a faulty butterfly vibrating with the air flow, which can also count for the 'boon boon' sounds your engine makes when the peddle starts to vibrate.

Lemme give you some tips to check on what might be wrong and how to pin point it..
Try reving the engine at idle, if THAT sound and peddle vibrations start off: It's most probably the Throttle Cable or the throttle body/butterfly.. ifnot try driving the car in different gears..

It's not essential that the engine makes a certain sound at a certain speed, in 1st gear at a certain rpm at whatever speed the air flow through the throttle body, engine's behavour and everything else is essentially the same that it would be in 5th gear at the same rpm. Try running the car in different gears all through the rev range, if the sound/vibrations are there at 80 km/h regardless of the gear/rpm: Then it's NOT the engine at all, it's something else..

I mean if at 80km/h in 4th gear the sound/vibrations are there and if in 5th gear at the same 80km/h the sound doesn't go away, you can rest easy it's not the engine.. Because if it were: By shifting from one gear to the other at the same speed what you have done is changed the torque load on the engine, changed the RPM, changed the gear and everything else, if the sound is still there it's not the engine..

I still can't say for sure what this might be, what i would suggest is that you hit the Hyundai dealership and let the techs loose on your ride