I suspect this could be compressor noise. At normal power settings with a ~20 'blade' centrifugal compressor at ~40,000 RPM the rotor-stator interactions are occuring at about 13KHz and propagating against a relatively high Mach no of intake flow. That frequency is sufficiently high that most people will not be sensitive to it. Higher frequencies also dissipate more rapidly as they propogate away from the aircraft.
If idle is 60-65% (guess), then the frequency will drop to around 8KHz, to which the ear is much more sensitive.
As to why it's more noticeable on the MU2, I don't know, but high frequency noises can be more directional in nature and sensitive to intake geometry and acoustic treatment (if fitted) more so than lower frequencies.