I have a Teledyne Laars JVH boiler furnace with 6 zones. When the thermostat farthest from the furnace calls for heat (approx 50 ft), the relay gives off a loud chattering noise. I've swapped the thermostat with another and the problem persists. The furnace is about 9 years old. Is this due to wear at the relay?

Look for a voltage drop problem. When relays chatter it usually means there is insufficient power for the relay coil to pull in and stay in during the called for event. You have a 50 foot run of probably very small gauage signal wire and that in itself can be a problem.