When my head gasket let go between cylinders 1 and 2 the engine idled very rough and knocked alot above 1000 rpm. I also thought a rod bearing had let go. Do a compression check. Disconnecting spark plug wires (carefully) one at a time while running should isolate any rod bearing problems. The bad bearing will stop knocking when the associated plug isn't firing. Volvos generally don't have any problems with sticking valves unless your timing belt slipped (another possible cause of your problem) and one got bent. Again a compression check should spot this.