I think you might be right about chariot noises actually. The Encyclopedia Britannica suggests that chariots, including four wheeled versions, were first invented in Mesopotania around 3000BC, which would tie in nicely with the estimated age of the toy. It'd be like having a model of the latest and greatest supercar, or possibly tank.
'My dad got to ride one in the last campaign against the mesonites!'
'Bullshit! your dad was shoveling horse shit the whole way.'
'Never said he was at the front lines. They let him ride the thing around before putting the horses up. Said if the campaigns go well maybe I'll get to have one when it's my turn in the army.'