My grand marquis engine sound changes when it reach 100 km / hr speed. It becomes more heavier and high pitch on reaching this speed.What could be the reason?

 

I''m not entirely sure.

does the car shift into overdrive at that time? This could indicate an overdrive problem. Is the rear axle in good condition? Are the bearings and differential well lubricated? Does the transmission have the proper amount of fluid in it?