1998 Mitsubishi Galant car has a vibration at idle

1998 Mitsubishi Galant mileage: 110,000. The car has a vibration at idle and when the engine gets to 3000 rpm (70 mph). We got the car from my mother in law who said that the problem started when she had the belts changed at 60,000 miles. I suspected the silent shaft belt wasn't set right so I changed out the belts and followed the procedure in the manual for the car. This didn't change it. I still think that the shaft is set right but I could not locate the hole to check the silent shaft position, as the diagram in the book is confusing. Can you tell me where this hole is supposed to be so I can check this again? Do you think it could be something else? I saw a listing in Service bulletins that said something about replacing the oil pump to cut down on engine noise. I really don't want to have to take this to the Mitsubishi dealer, as I know if I can find an answer, I can fix it. Thanks for your help.