Well, I´m far from being an expert of musical issues since I´m far for being a musician... but I can help you with some SigurRós discography issues, even if most of their songs are slow, peaceful and deep.
First of all, we can not say there´s a lot of instrumental sogs o their catalogue. With that in mind, we can eliminate all the instrumental songs based on eletronic noises or distorted strings. But we can still eliminate those songs with much more than two notes.
So we can keep just a few alternatives for you to search by name:
"Smáskífa" A &/ B
"Fryst ferd" (maybe the most obscur of this list)
"Ba ba" &/ "Ti ki"
Good luck. Hope you can find... tell us later