As documented by Wikipedia, guitarist Jimmy Page said:

There was an awfully squeaky bass drum pedal on ‘Since I’ve Been Loving You’. It sounds louder and louder every time I hear it! [laughs]. That was something that was obviously sadly overlooked at the time.

Rolling Stone ranked it as the #36 Led Zeppelin song, saying:

Page spent months on his solo to this epic – then settled on his original demo. Good thing: The slow blues is one of Zeppelin’s most soulful moments, Page’s guitar veering between spare and raucous attack, Jones playing blazing organ and Plant shrieking pure heartbreak.