You mean a dpf? Diesel particle filter
The 170's get clogged up a lot with short journeys/town driving. Not a huge problem, just a design flaw
The revs will then hold out longer to keep the exhaust temp up to burn off the crap in the dpf that's built up
A motorway blast every now and then clears it out. Alternatively, a lot of tuning companies make dpf delete pipes which then requires a remap quite convenient really!
My old 2.0tdi Audi A3 170 kept getting clogged. Just went for 10mile motorway journey once or twice a week to clear it