Seem to recall a similar thread on here a while ago where the noises on the ground were discussed. I seem to recall that it was 146 & the smaller Airbuses A320 series that were the main offenders. I believe early flap extension noise not engine noise at all.

I often work outside not too far from your position and am able to enjoy the "entertainment" overhead on fine days. I thought that aircraft stacked at Ockham not Epsom - not too far apart - but that's just the pedant in me.

Time to try the search function.........

The noise may be caused by an exhaust valve which opens when aircraft are descending and the pressure differential reaches a low level. You can also hear it in reverse when ascending. You can hear it clearly whilst on board an MD80 as it climbs. I think this exhaust port is open on the ground and it closes as the pressure differential increases with altitude.On the way down it opens as the pressure differential reduces to a low level. In flight at altitude it is normally closed. As far as I understand it's an exhaust that dissipates warm air from the electronics bay.

Could it be the apu starting up?

matblack, you could be pulling one's leg here but incase you're not; at 5000' most jets will have a cabin altitude above aerodrome level, will be at low power and it's unlikely that the exhaust (outflow) valve will be open and the idea is to pump air into the cabin to cause its level to decrease to be at aerodrome level by landing.

Smug.
 

:ok: I must admit I'm feeling a bit smug. I really thought someone would straight away come on and say; 'Stupid boy Rog', its the Internal Change Over Low Pressure Flange operating - duh!' :rolleyes: But they haven't. :)

I like the '757 slats opening' momentary resonance - that could be it and there are enough 757's, Lord knows. Is that about right for distance from touch down?

Mark 1's 'Compressor blade interaction tones' are interesting, because it seems to me the noise is preceded by a short, quite quick 'down doppler' engine effect, very suggestive of slowing rapidly. I always think of it as 'dropping out of warp', but there is an element of 'hoot' about it.

Keep the ideas coming guys, lets see if we can get a definitive answer, because I really want to know. :)

Roger.

Is anybody else thinking of standing under a flight path this weekend to find out just what this noise is? I hear the weather in the South East is supposed to be lovely...

I think it is the engine spooling up briefly. The RB211's having 3 stages tend to make that growl. Watch video's of L1011's taking off, and tell me if that growl is the same sound you are hearing.