Dan,
If you have received 20 years of service from a transfer relay, you have done well. Esco makes a quality product so I would recommend staying with the official replacement that they suggest. It is a straightforward installation. The label on the inside cover identifies the shore, gen and load center wiring and respective colors.
FYI, what damages the relay contactors and causes the buzzing is transferring large amount of amperage loads from one source to another. (I am not implying that you did that). Hope this helps, Fred