It's from the truck, it's the transmission of the truck which is evident by changing pitch as the truck downshifts while slowing down.
It likely uses a transmission as a tradeoff between torque and RPM of the electric motor or because it's a simpler task to retrofit an existing truck driveline with an electric motor bolted onto it.
These are so noisy because of the use of straight cut gears, which you choose when preferring strength over noise and comfort. Most regular cars use helical gears which reduces the noise due to continuously meshing between gear teeth, whereas straight cut gears are like a paddle wheel, so the teeth are smacking into eachother head on each time they engage. That is also why they clunk when getting on and off throttle.
We have some similar trucks running around Australia at the moment and the sound is the same, I'm also familiar with the sound from motorsport!
Some cars have 2 gears as well. One for the acceleration and one for cruising efficiency and top speed. So my guess is to get it going easier and then have 2nd as the eco gear for the speed they cruise at.
I am unsure sorry, electric motors do still have efficiency curves so that could be it. But I think a retrofit to an existing driveline makes more sense.
Food for thought though, trucks are much much heavier loaded than cars. 100-150+ tonne vs like 1.5-4 tonnes for a car. Cars are trivial by comparison.
Perhaps the motor they have available doesn't have enough torque for it without gearing down.
It likely uses a transmission as a tradeoff between torque and RPM of the electric motor or because it's a simpler task to retrofit an existing truck driveline with an electric motor bolted onto it.
These are so noisy because of the use of straight cut gears, which you choose when preferring strength over noise and comfort. Most regular cars use helical gears which reduces the noise due to continuously meshing between gear teeth, whereas straight cut gears are like a paddle wheel, so the teeth are smacking into eachother head on each time they engage. That is also why they clunk when getting on and off throttle.
We have some similar trucks running around Australia at the moment and the sound is the same, I'm also familiar with the sound from motorsport!