I have a house built in 1970. The double hung windows are now hard to raise and must be propped to stay up. There are no weights or chords. No visible springs or mechanism. No openings in the glide on the sides. Just a multi-profile solid metal strip on the sides. However I heard a "sproinggg!" like a spring breaking inside somewhere and now they are hard to raise and fall easily. I don't even know what to call this type of window so it is impossible to search on the net. Any info would be nice. notice attached pic. Thanks.