GWR 2-8-0 3802 - Name this sound

[Daytona]

Does anyone know what the warbling? sound from GWR 2-8-0 3802 at 6-11secs in is ?

 

[John Webb]

Possibly the cross-head operated vacuum pump?

 

[The Crab]

Possibly the cross-head operated vacuum pump?

Sounds to me like something binding, can't think what but it does stop after 3 or 4 "warbles",

 

[Flying Phil]

Could it be a brake block not quite fully cleared ?

 

[randyrippley]

The warble coincides with the exhaust blast. Is it something in the exhaust pipework that's acting as an organ pipe?

 

[Ashley Hill]

Ive heard that from western locos on my line. When I've asked about it nobody on the footplate seemed to think it anything to worry about. To me it sounds like it's something to do with the pistons.

 

[Irascible]

Given it's once per rotation & it stops, I'd put odds on something to do with the vacuum pump too... be nice to know for sure though.