Maybe I'd a bit naïve, but I would not be expecting to see any nasty stuff running back out of any of these fuel lines, unless you have accidentally dropped something nasty into the tank, or the electric lift pump in the tank is shedding stuff?

 

Edit:- out of a bit of laziness and concern, I never did replace the filter on a 2002 Polo 1.4 16V over its 13 years life from new and 103 or 105K miles, so maybe I would have seen something nasty if I had replaced that filter. The filter on my 2000 Passat 4Motion got replaced at maybe 6 years 50K miles and there was no evidence of any nasty bits when it was disconnected.

Edited 18 November, 20187 yr by rum4mo

Letting people know that this issue is now sorted, it was the inside fuel line tapping up against the wall of the channel it was sitting in so I just moved it over notch on the rubber grip so it has space. 

Now complete silence has resumed.