Thud...check trans, cruise control?

[OnTarget]

Three times over the past two months I've had problems with the Passport doing the following:

Always when stopped at a red light, in drive with foot on brake, I feel a thud and look down to see the check trans light and cruise control light on, then shut off about 2-seconds later.

After the first time, I happened to be having some service done at the dealer and mentioned it. When I got the car back they said everything checked out fine and they had no explanation for the occurrence.

Has happened twice since then, averaging it out it's once every two weeks or so.

Any ideas?