A: It’s likely that the nut slot the string passes through is too narow for the string gauge you are using. The string is binding in the nut, then snapping loose, making the “ping” you hear, along with causing the tuning problem. One thing to try is to lubricate the nut slot; graphite from a pencil works well. Just lift the string out of the slot, work some graphite into the slot and replace the string.
If the slot must be widened, this is a job for an expert – have a qualified repair person check out the nut and make the repair.