The truck is making a sound that is quite high-pitched, and described as a whine (it has roughly the pitch of a human voice). But the boots are making a very high pitched sound, described as a "squeak". You can hear the boots even though the truck is noisy, probably because the sound the boots make is high pitched
If you hear A over B, it means that A can be heard even though B is noisy.
What we can't tell from this fragment is why the boots are squeaking, or why the truck is whining. It may be that the boots are new and the leather is rubbing, which makes a high-pitched sound. It may be that the trunk is at high "revs" which makes a "whine". The truck may be far away, which is why it is not too noisy.