fahad bro can u kindly explain the highlighted point? i didnt get it! what sound can one hear and how is it related to tappets?
btw, tappets adjustment are a very controversial area... our respected member @vague_one (usman bhai) , even though hes SUCH a big car enthusiast, he NEVER got his idsi's tappets adjusted or throttle cleaned... NEVER... and i ve been with him in his car and its flies like anything.. it was on petrol though, totally... still... but then, he is magical, may be he cast some spell on his cars 
one more thing, i dont think slightly out tappet adjustment would make the RPM unstable or delayed starting IF and only IF the car is on petrol... CNG pay tou intake k pass say makkhii b guzar jae tou check light jal jati hy or RPM drop 