It could be a wheel bearing, cv joint, or control arm bushing
there's not always a simple answer to some questions like this. the easiest place to start is with the tires and wheels. was there any excessive wear at all? sometimes one tire isn't making contact with the ground while accelerating and decelerating on an incline in the dirt. another possible cause of such noise may be that your car's alignment needs adjustment (which would also predict why you had more problem when driving over hard surfaces), that both front wheel hubs need replacement (if you hear it from both sides), or that certain bolts are loose on one of your front axles which is usually more common during winter months due to cold nearby terrain.
why is my car making a knocking noise underneath?
it sounds like something is loose and needs to be tightened.
under the hood, when there is a humming or knocking sound when you're driving, one of the first things that should be checked is your engine mounts. that's because if your mount bearings are in need of replacement, that could explain why you're feeling vibration under the seat and annoying noises when your engine spins too fast (the noise changes rather abruptly). if they happen to be broken or missing altogether out of place, then it will absolutely lead to this kind of symptoms that you experience while driving.