[Resolved]  Toyota Corolla Altis — Vibration sound from the dashboard

Dear Sir,

My father purchased a Toyota Corolla Altis in November 2008.
1) The car was running fine untill we found out the dashboard emits a vibration sound when the car is at the 2000 to 3000 rpm range. below and above that range there is no sound. What could be the problem?

2) The fuel economy indicator in the car is never constant. On the expressway it shows an average of 99 km/l and within a few seconds it changes to 34 -35 km/l at the same speed. What could be the problem and how do we find out the average.

3) Our car also comes equipped with the distance to empty meter ( the distance which can be travelled within the available fuel) . Is it very accurate or is there any buffer margin? If yes then how much more can the car travel as compared to the indicator readings.