Engine stress at high speeds

[banpei]

Maybe you could compare the noise the engine makes with lower gears. At 120km/h your engine should be doing 4200 rpm in fifth (with the OEM 4.3). This should roughly be equal to 90km/h in third. So if your engine makes just as much noise (and is shaky) when cruising at 90km/h in third, it's the engine. If it's the engine, perhaps it's the T-VIS opening and closing regularly as 4200rpm is when it gets actuated by the ECU.