Old is not a bad thing for speakers....
1) grab a screwdriver, remove sub from box. Check box for anything that could cause rattling (lost/loose screws, hair balls, etc). Check the port (if there is one) for obstructions.
2) remove 'weatherstripping' from sub, grab some compressed air and blow the cone, surround, and voice coil. Make sure all dust is blown out.
3) run a bead of silicone around the cut out hole and replace speaker. Make sure to use just enough silicone to seal the box and not run out onto the speaker. Screw the mounting screws in tight. You can also add a very small amount of silicone to the screw holes.
4) make sure all connections (audio/electrical) are a) clean, not corroded and b) tight. This includes from the device sending the signal.
5) make sure no audio wires are crossing electrical wires.
6) Plug sub amp electrical into another outlet on a different circuit.