My transmission has just started making a howling
If the problem just started out of the blue, then my guess would be a bearing. Make sure you have the proper oil in the box which is the 600W oil. Sometimes people use 90wt gear oil and that is no where near heavy enough. That will cause noise as well. Since you are going to have to remove the tranny from the car to replace the bearings, give it a good inspection once you have it out.