I have the same thing happening to my Strat. Mostly on the G string though. On my guitar, it's coming from the part of the string that goes between the tuning post and the nut. Possibly the nut can have be better slotted, I guess. I have a different neck than the original, but I had the same thing happen on the original neck.
I tried putting some dampening material on the string above the nut. It took away that "PING" sound, but it deadened the string quite a bit.
I've gotten used to it. The B and E string have string trees, and this doesn't not happen much with them as a result.