Dav, another suggestion is to develop a noise map contour by using an array of microphones and testing the engine at different rpm have developed and used this technique to support many OEMS during engine development.
It really all depends on how accurate a picture you want to get of the noise sources and their contributions.
Do you have a specific problem at a particular speed? Is the noise low or high frequency? Where are you measuring the noise. Is the truck fitted with a cab, steel frame and glazed? Does it occur during a particular operation, e.g. under load from the steering or hydraulics. I recently had a vibration problem on a GM V6 when under steering load and it was the torsion damper on the d/shaft but under normal load it was OK. The noise could be caused by a worn or out of balance part. Have you had this problem on other machines of the same model. These are just some of the things to consider. Does it happen when the engine warms up, I had a problem once where a noise occurred when the engine was warm and discovered it as due to a modal shift in the exhaust system when the exhaust rose above a certain temp.