I will have to go a bit more in-depth on interstate, but if you've heard the "vocal only" track that's on youtube, there isn't anything, but I'm guessing it's on another track.
I think the 'whistle' in Plush is just chord bending. there is more than one guitar track used on plush, so you're hearing one guitar over another.
the 'whistle' on meatplow is another 'ghost' where you just get some feedback too, it makes for an interesting sound.
You can hear Scott in VR's The Last Fight snorting...or maybe it's not Scott, who knows at 3:09. You can really only pick it out with headphones on.