I've started messing about with open mic recording and am having a gas with
all the usual humming, swearing, neighbour's lawnmower, birdsong etc.
accidental recordings but I'm also getting a little thump once in a while
which looks like this on the recording:

http://www.apah69.dsl.pipex.com/keith_g/show/glitch.jpg

It's very likely the thermostat or boiler which are both brand new and
therefore quite silent, so I haven't been able to catch it 'at it' yet but
what's mystifying me is why does it drag the waveform down which then takes
about a 1/4 of a second to recover? It's nothing like a scratch (or the
tower of crap the old boiler used to add in to the proceedings) and
therefore damn near impossible to remove!

Anyone got a clue what's happening?