I may be off-beam here, I've only seen the ad. once, but I suspect it's the music used in '28 Days Later'.
As to which bit it is - it maybe the track 'East Hastings' by the band Godspeed You! Black Emperor, which is in the film, but not on the soundtrack album because the makers were only licensed to use the music for the film itself.
The soundtrack does include other music used such as Brian Eno's "An Ending (Ascent)" which is used frequently in the film.