How does he get that gurgle thing, like a bass chicken?

[Walter Webb]

Is it vocalization, splatty spittoon shots in the mouthpiece? What? I don't think there is any wah wah pedal or electronics here.
http://www.youtube.com/watch?v=eN2_itMXRF8&NR=1" target="_blank

Here, the riff starts at 0.52 sec.
http://www.youtube.com/watch?v=RJfiYdQcQtc" target="_blank

[ginnboonmiller]

Haven't heard the second clip yet, but the first one is definitely electronic - an Envelope filter, which changes the cutoff frequency depending on how loud the signal is. They're tons of fun with the tuba.

If you want to play around with one, the DOD FX-25 can be had on the used market for $25-50, and worth every penny.

[Jesse Brown]

That's Oren Marshall, this is his website: http://orenmarshall.com/" target="_blank, you may be able to contact him directly for an answer.
If you get an answer share it here! I'm interested to know exactly what he uses. Ginnboonmiller is correct about the Envelope filter, it seems like he may have a contact mic on his mouthpiece (it looks that way when they zoom in for the trombone solo.)

Thanks for sharing!