I know what sound your talking about I have a 2022 MT,
It is a slight high pitched wrrr or as you say whistle when you at highway speed while barely accelerating or decelerating,........
it sounds for like 2 seconds.
Then quickly goes away as it came,.... it will go away instantly if you power up or let off the gas as well.
Its normal, its the meshing of the gears at no load coming of a load (load meaning under power)
A manual transmission has a lot going on in that box and it's just the shift boot between your ears and the gearbox........
Not much sound insulation between the gearbox and that shift boot (Toyota could have put more in).
The boot turned inside out,...... white material is sound/heat insulation.
As you say if you do nothing and keep speed and throttle the same it quickly goes away,...... gear oil is doing its job.
In the older "days" truck MT's sounded like a "school bus transmission"
They Wrrr'ed and sang like all get out...... Normal for them for their ratio and gear oil type.
Easy fix ,,, multiple choice,....
A ) Never slow down,.. just keep shifting and stay on the gas
B ) Turn up the tunes
C ) Tell your wife about your new girlfriend while driving to the store
D ) Feel good that you are in the 1% that get to drive a MT ,.... because it is happy and singing to ya!
Trust me pick any one of the above,..... BUT not "C".
