Had the same problem on mine,turned out to be the pickup and return pipe's on top of the fuel tank sender unit,pop the round black cover off which is under the boot carpet and I'll bet the pipes are rotten and its pullin air in through em,the area seems to collect all sorts of C**p,leaves/mud etc,it seems to be a known problem something to do with sh***y quality electro plating on the cover,I drilled the old ones out and replaced em with 316L stainless pipe,better than paying £72 for a new one only for it to start all over again in the future
P.S Is it a pig to bleed up and does the back end around the toe bar/rear cross member have an aroamer of diesel,thats how I sussed it,the return pipe was leaking it out into the little well the sender unit sits in
P.P.S if it is the pipes,the union's are a rite T**t to seperate from em,they will need a bloody good soak in duck oil/plus gas,wouldnt bother with Engineer's aftershave (WD40)