I heard somewhere that the whistling wasn't supposed to be on the song, but Redding commonly whistled as filler until he came up with lyrics.
since he died, the song went out as it was.
"I mean, I got this call on a Monday, and of course Otis' plane went down on a Sunday morning," Cropper told NPR back in 2000. "And they said, 'Get that thing finished and get it to us.' So, I went to work on it. And probably the music is the only thing that kept me going."