This was not on my Buick, but my Grandmother has a '98 Chevy Malibu and she recently had some loud vibration coming from her glove box/dash when the A/C was turned on. Low and behold, when she took it to her local shop it turned out it was a dead mouse stuck in the blower motor.
Luckily it wasn't in there long enough to start to smell, but it did damage the motor and the fan.
