On ZZ Top, there is either flanging or phaser being applied to the lead vocal.

here's how I would apply it:

1. Copy your lead vocal track to a new track (This is just for safety sake - your ultimate 'undo' function)

2. Mute the original, and apply a flanger (or chorus) effect with the rate set to less than one cycle per second (less than 1 Hz), keep the depth on the flanger effect pretty shallow, but make the mix 100% wet or nearly 100%.


See how that works for you.

Alternatively, try the same with a phaser effect. The key to the slow liquid sound is to keep the rate slow, and the depth shallow.

-Scott