I used to have an old copy of Metallica's black album that would warble, or wow, at quite a few spots after I'd listened to it a gazillion times. I'm not sure if it was because of the tape deck in my old van, or the tape itself, but there was a varied stretch in the pitches of certain songs that I thought sounded really cool. This wow effect on the ending guitar solo in Unforgiven and on Nothing Else Matters just added a really cool element to those songs. I'm wanting to reproduce this wow effect on my guitar and my bass.

Anyone have any clue as to how I could pull this off?