Yup - had this same problem.
The 'sound' is a low-pitched rumbling/growling - a bit like a bum note on an oboe.
I simply poured the contents of a boiled kettle over the external, small diameter white plastic pipe (it should drain into a nearby drain) and haey presto, now the pilot light LED stays on after I've pressed the reset button and the boiler is functioning perfectly.
While you're fiddling with your boiler, take the time to check the water pressure - it should be between 1 and 2 bar.
The Kettle King!
December 2010