If your dpf was blocked the car would be noticeably down on power and most likely throw an eml. I take it the noise is coming from the front of the car since you think it's a dpf issue..?
As you say it's only an issue for the first twenty minutes i'd be looking at items related to the warm-up cycle - glow plug relay, for example.
As you say it's only an issue for the first twenty minutes i'd be looking at items related to the warm-up cycle - glow plug relay, for example.