If what you heard/felt was a vibration it was not the common "pod nod", which is more like the a subdued version of the lurches you get on the underground.. .In hot climates, probably your "second sector",. .the F/E has a tendency to send the ram air doors a little too wide open, and depending on how open the ram air doors are, you can get a rumble, rattle, or chugging. At some openings it does feel like vibration, and is usually reported by F/As at L1 door.