Hey, I noticed soemthing while I was listening to the live version of "On the Turning away", from Nassau Coliseum. Here's a link to it.

At 5:00, he plays the melody with the guitar sounding like it's mouth it open, if you guys know what i mean. He plays a modified melody after that for a comparison. It sounds like some kind of talkbox, but he's lacking a huge tube in his mouth, and you can see his feet clearly while he's playing it and no pedals were stomped.

How does he make the noise? It's obviously in the fingers, I just can't find it!
Pretty sure that's a wah. There's some harmonics in there but I distinctly hear a wah, first cocked and then manipulated, in that section. 
Roc8995 You can see his feet when he plays it. And I don't think I've ever seen or heard David Gilmour use a wah aside from Echoes, which he used for the laughing sound. Might be wrong on that though.
At 5:07 you can hear a wah sweep, clear as day, but his feet aren't in the shot. I don't mean he's working it the whole time, but it sure sounds like a wah to me. It's also possible the solo was dubbed in later, if the live one wasn't up to snuff. 

There are some harmonics in there that jump out a little more than usual, either because he's got a cocked wah going or just using a fuzz/OD set to emphasize those frequencies, but I don't see any way around what sound to me like subtle but unmistakable filter sweeps in that solo. 

A quick google of Gilmour's studio and live boards shows that he's pretty often got a wah somewhere in the chain. Some of his early Cornish boards even had a forward/reverse switch below the treadle, so it's obvious he intended to use it for more than just the backwards seagull sounds on echoes. 
I'm with Roc, mostly. Probably some kind of wah is the most probable answer,

It could also be done with some kind of programmable auto-wah type effect. That would be my next guess. But AFAIK, DG has never used one of those.
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