Hi mate, i have had the exact same problem with my mk3 fiesta (haven't got round to fixing it since easter cos at uni at the mo).
yeah the hair like fibres have come from the packing in the box which sounds like its collapsed and blocked up the pipe. so guess you got two choices, cut the box open and pull out all the packing (like im gonna, but it might be a bit loud) or buy a new exhaust section to replace it.
hope this solves your problem
