Not a lot to go on there, like saying to a doctor, I've got these spots appearing on my arm, what's up?
I don't want to insult your intelligence, but as you've only been driving a 3-pot diesel for a fortnight, you may not yet have got used to it's characterful sound.
Also, it's not a good idea to change down too early, as per a petrol engine.
If you give it a bit longer and then post some more details, I'm sure that someone will be able to give you more of an insight.
Cheers,
Mike