Whenever the engine of this bus was up to temperature, the hydraulic fan will be driven when braking, which slowly takes over the speed of the engine (so not directly connected but A small delay, and if the engine gets even hotter you should also hear the fan when you accelerate, but the fan will also have to have a limit I think if the engine runs faster than 1800 rpm Then don't think the fan will spin faster, this certainly adds something You should be able to hear the fan especially outside you can hardly hear it anymore when the engine has passed the 3rd gear and it still brakes, I am now talking about a ZF gearbox for example.That plays a small role, but the principle is that the fan rotates with the motor with a small delay. Hope you like this idea and I don't think it will be a lot of work to add that one mod I have some sounds myself if we need to add the fan sounds.
aa if you fast forward to 2:00 you will hear the beautiful sound of the radiator fan very cool i think we can reach this in the middle i'm just not that good at that.