The late, popular folk singer Nick Drake wrote a song titled "Saturday Sun" which appears on his album "Five Leaves Left".
Samples/Lyrics
Throughout the song, there is an underlying drone sound. It bears a strong resemblance to the drone heard in the cabin of an airliner during travel. Given that images of airliners appear in the album artwork for The Campfire Headphase, and the following track is titled "Slow This Bird Down", it is reasonable to speculate that this sample source is accurate.