I have a Opel Vectra 2008 1.9CDTI caravan. It has 177 000 kilometers behind. I got the gearbox changed to a refitted M32 at local garage. Before the gearbox was changed i got the clutch and DMF replaced at the same garage. The original gearbox failed after 50 kilometers of driving with the new clutch installed.
Before and after all these fixes i can hear strange buzzing/humming/rattling noise coming from the engine compartment. The noise does not sound that bad and you really need to focus to hear it.
The noise will appear when i press the clutch all the day down. Giving the engine some revs will stop the noise, but right after the revs come down to normal idling level the noise comes back.
Engine temperature does not seem to affect the noise.
EDIT: Yesterday (2.5.2018) i visited the local garage where the clutch and gearbox services were made. The mechanic located the sound and said it is most likely coming from the water pump or the high pressure pump and it is nothing to worry about, as i have warranty for all the new parts plus the whole car itself. He also said it should be common for a revised gearbox to make some noise. I'm going to take the car to a certified Opel mechanic just to see what they think and to be sure.