I can't be absolutely sure without measurements, but it will be the capacitors. Those are electrolytic capacitors and their capacitance reduces and their ESR (equivalent series resistance) increases with age. The relay is 24 V DC, so the coil is run from a rectifier, and the capacitors are there to smooth the supply. Without a smoothed supply, the coil current will dip a lot every mains cycle and lead to buzzing.
Relays contacts can wear out but the coil doesn't. The coil is DC, and a relay with a steady DC current will never buzz.
Transformers last for ever unless they get overheated.