Triangles. Trusses are divided into triangular shapes. The webs should all be the same when you sight down the centers.
Framing members from the ridge down to the ceiling joists.
Tight joints and gussets. Gussets were often plywood in the 60s.
Maybe a knee wall down both sides of the attic. Make sure that it hasn't shifted or moved in any way.
Look for cracks and splices, blackened wood and soft wood.
If you go to the attic and the ceiling joists sag and follow the same contour as the roof, you may have foundation problems or other support issues.
It could be where the house has been repaired or added onto.
It could be anything, look around, spend some time and take a straight edge or a string with you. It needs to be 4ft to 8ft long. A string is easier to move around, but you will want someone else to help with a string. Check parallel to the rafters and across the rafters. Check the ceiling joists. It may not be anything serious at all.
Tom in KY, hoping you have good luck.