Hmmm, Frequency varies as inverse of length... where is the car?
= bridging point. Put a significant mass on a guitar string and tell me what note you get.

Ignoring harmonics - I would bet there are several. The base note as the mass moves (though it's mass would alter it a bit), the one based on the free length above, and one based on the note below. I think we would have a glissando too as the car moves.

And then there are the sum and difference harmonics (beat frequencies) because the system is probably non-linear. And then all the harmonics of all frequencies mentioned and beat frequencies of therof.

Buildings make noises in high wind and not that pleasant sometimes. A road sign near me has no top to the pole (a tube) - and that moans in the wind. Same principle.