Last September 2014, I change oil from 20w50 to mobil 15w40.. The Corolla 1999 2e engine runs smoother, feels lighter and quieter..

Yesterday, I change it to mobil semi-synth 10w40 .. At first start after the change oil, the engine has a slight knocking sound .. I haven't had a thorough check at the knocking sound as of the moment because I haven't use the car since yesterdays change oil ..

Could the thinner viscosity of 10w40 cause the slight knocking sound ?? Should I change it back to 15w40 ?? Will my engine suffer damages if I continue to use the 10w40 ??