Keith G wrote:

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?

Clearly you have some interference. What you really need to do is find out
how it is getting in. Replace the microphone with a fixed resistor equal
to its outpout impedance. If the thumps go away its the mic and there's
probably little you can do about it. If it still thumps then its further
down the chain. Try connecting the same resistor straight across the input
to the mic amp. This will eliminate the mic cable. After that you are
into thumps getting in thru the power supply.

HTH

Ian