Phil,
If the howl only occurs in the indirect gears, it is probably coming from the input gear and it's mate on the layshaft. If the howl is also evident in top gear, try adjusting the number of shims between the final drive and the main case (it will be a case of trial and error as Douglas never gave any instructions on checking this). Also, with the final drive removed, check that the large nut that secures the bevel gear on the shaft is tight - it's not a good set up as the thread is on the splined part of the shaft. If it is loose, check the spacer between the bevel gear and the bearing - any wear here will affect the mesh of the final drive.
Regards,
Eddie.