This happens because your mechanic is not qualified - a person who buys a set of tools to change oil and spark plugs is not a car mechanic. The noise you experience is "seen" in a torn down gearbox with a dial guage and adjusted with shims which are sold as genuine spare parts.
in urdu - its called - kontershaft ke guchchay mein chaal aagaye hai - shim washer se bithani paregi, experienced mechanics know of this.