Had this problem with the my last few cars. Think my problem is amplified with the fact that I have to park on a hill, requiring more hand brake to hold. As previously mentioned, moisture seems to increase the problem. If I forget (or don't have time) to take the car for a spin, after washing the car, it can really 'clonk' as the brakes free up from the discs. Not sure if new brake disks will help, just a trait.