Them shocks may just be dead...
...but,.. the sound, is from the shock damper... that doesn't make the shock return from compression, it dampens the return...
The springs make the return...
Can you post us a vid?... of the you getting on and off?...
If it compresses every time you sit, but never comes back up at all... I have to imagine that by now, the fender is sitting on the tire...
...
Also,.. please measure, bolt center to bolt center,... the shock length at rest, without you on it...
I have the same shocks, and mine are about 12.2"... if I remember correctly...