The first thing to check with any Japanese camera from the 1990s and earlier is the foam. Open the back and run the tip of your little finger along the edges of the back. If it comes away with a black line, the foam has to be replaced.
It takes about 20 minutes to remove and replace the foam. Lighter fluid will ensure that you remove as much of the old foam as possible.
And like others have mentioned, be sure to replace the mirror bumper in the mirror box.
Try to avoid letting any of the old foam fall into the shutter assembly.