The Kestrel had rotors, didn't it? I mean the old Halo 2 concept. The bird on the back and the picture of that Kestrel look similar.
This may finally be the Kestrel Light Troop Transport.
The only explanation I can think of for rotors over VTOL turbofans is that they're capable of utilizing ground-effect at low-altitude, which is likely their running altitude, since they're carrying 3 SPARTANs each. That's a ton and a half, on average, of extra weight, plus equipment, plus fuel, and it's all in a small vehicle. If they're within about half of the total rotor span (total span of all 4 rotors), they can get more lift, if I'm remembering correctly, meaning they can carry the weight with better effect. None of the aerial vehicles thus far in Halo, save the Longsword and Shortsword bombers have had much wing area, and have all relied heavily on a thrust-to-weight ratio greater than one and VTOL thrusters. All of the TT craft are in that category. Rotors actually create lift, instead of using thrust to stay afloat. Saves fuel, too.
Er, I'm not a helicopter pilot, so I can't really say for sure whether or not all of that's completely accurate, although I know it is for fixed-wing planes.
I hope it's the Kestrel. It'd really bring closure to that matter, once and for all.