if it's rumbling, it must be the final drive ( bevel ) bearing, not the paralever bearing, that most people have mentioned .
i posted a while ago that my bearing supplier had sourced me an upgraded bearing, this cost £46 trade price, it's been inserted in a bike that used to knock out the usual bearing every 8-12k miles, the bike is chipped, with all the go faster goodies on it, plus an ADV low 1st gear box and an 850 rear bevel, this bike beats anything away from the lights and gets thrashed despatching, thats why the rear bevel bearing get knocked out so fast. Anyway to the point ; so far the "upgraded" bearing has done 7k miles, so at the moment its too early to tell if its made any difference -- so not much of an answer really, but i'll keep you all posted how long it lasts.