Under bonnet air intakes don't help engine performance, when they are running at full torque on hot air. You can hear it come off song as it climes a hill with no blower or turbo to help. Air forced in by the cooling fan does help to keep the pressure up under the bonnet but that is hot, making the problem worse. Domed air filter intakes design's tends to lower the pressure in the filter body as well, as it is trying to get air from a flow going past at 90 degrees. I know its not standard but if you replaced the two intakes with small forward facing short bend intake's with a wire grill across the opening to keep out any debris it will make quite a difference to its performance on grades, helping its breathing, and the man looking after the gear changing. Just my thoughts, Its a long time since I have seen a proper rig like that doing what it was intended for. I am presuming it is a Roller making the noise, they need cool air for best performance. Keep the temperature down on the heads and all of those little grommets will last a lot longer.