Engine humming sound enter cabin.

I have read quit a number of people talk about this topic but never really seen anyone send the car to proton and find out the root cause.

Here is the scenario, 1.8MT when you start up the engine running idle for awhile then you can hear the engine humming sound very clear. Car is not moving so it can't be the tyre. air-cond and radio both turn off too.

I have check other friends inspira included proton test drive car. No such behavior.  So I believe only certain unlucky car is having it.

Has anyone out there fix this issues and would you mind share some knowledge about this.