I had the same problem for many years - it was traced back to when I was a child and inserted a battery up my nose. I only resolved the problem when I sneezed it out - sadly it was launched from my nasal passages at a frightening rate and struck a friend I was speaking to at the time directly in the face. Luckily he survived but now has a terrible fear of sneezing.

Its probably due to static build up - wear less polyester. Also check your cars floor mats - if these are made of manmade fabrics as you move it builds up static charge that is released when you touch a metal object and gives a little shock.

Qatars best known Pirate - Yaarr!