Q: Police interceptor makes grinding sounds

My car has 160,000 miles. Recently, I've noticed that buzzing and grinding sounds started coming from the Police Interceptor at RPMs around 3000. Also, when I am driving on the highway, there is a significant shudder during acceleration. This occurs only when the transmission is not in overdrive. The reason for that might be that I manually disabled the overdrive. It is either that or because it downshifts out of overdrive as I give enough gas. The transmission was rebuilt. Besides that, I changed oil cooler, condenser and transmission cooler because the transmission fluid was leaking. I cannot hear the sound when the car is parked or in neutral. This problem occurred suddenly. I did hear some quiet buzzing during driving last month, but this grinding sound occurred very recently and it could be heard occasionally.