Does the compressor run? It's normal for older relays to rattle when you shake them. Test with an ohm meter. The start and run terminals should have infinite ohms when right side up and 0 ohms when upside down. Check your compressor terminals. Top terminal is common, right is run, left is start. Check from common to run, common to start and run to start. Typically, the start winding has more resistance than the run winding. The start to run winding measurement should equal both common measurements added together. Check all terminals to ground, should be infinite. Any of the generic relays should work.