Thanks for your response.
When I was flying, there was no throttle sound when the left stick was at the bottom. It only start to sound and elevate when the left stick have passed the middle position.
I tried selecting/deselecting the throttle zero button but it does not solve the problem.
I understand this might be a weird issue. I guessed I will live with this issue. Not much a problem for me since I am still able to fly. Please do let me know if you come across similar issue and had a new fix for it.