Mazda 626 makes noise at highway speeds

I have bought a Mazda 626 2002 year with 72k miles on it a few months ago. I could not notice while I did my test drive but it makes noise when it exceeds 40 mph at hihgway. There is no noise at low speeds. I am not sure where that noise comes from.

Do you think it is a serious thing that I need to get it checked and what might be the cause?

What kind of noise??? Thump,grind,whappa whappa,ding ding ding?

It is hard for me exactly to define it but it is like a whappa whappa noise.

Could be very serious–do not drive over about 25 mph until you have this fixed and do it tomorrow (monday).

I would check the tires first. They often are the culprit of such noises or show what other parts of the suspension or steering are at fault. Does the car steer ok? Stop ok? Does the noise speed up as you speed up and slow down as you slow down?

I have put 3000 miles on my car after ? bought. yes, the sound kind of speeds up and disappears when the car slows down and reachs under 40 mph.

but ? kind of feel a weird thing when I speed up so much like over 70-75 it disappears again maybe it is still there but ? cannot hear it due to other noises. There is nothing wrong with my steering or suspension or brakes. I got my car checked at firestone and they told me your car is in a great condition.