The last week or so, the temperatures here have dipped down to below -20C (-5F) overnight and in the mornings. I was driving to work the other morning and I heard this fairly loud "thunk" from the side of the car. I remember having this same thing happen last winter but I basically forgot about it over the summer. It almost sounds like a rock or big dirt lump or something hits the side of the car, however I know that's not the case and it happens frequently but only in cold weather.

Has anybody else experienced this? Any ideas what the sound could be?