Have a Bryant Plus 80t energy efficient Gas Furnace that is fairly new (@ 2 years) that fires off and heats correctly, but when it reaches temperature on the thermostat, shuts off with the relay clattering as if bouncing on its contacts. Question: Is it dangerous? What relay is it? Is it on the control board and does the whole control board need replacing? Looking at the schematic for the circuit it looks like it might be IHI/LOR or the Inducer motor Relay (SPST), or MGVR-Main Gas Valve Relay (DPST). Comments?

First of all that unit has a 5 year parts warranty only if the work is performad by a Bryant dealer.
Many times this is caused by the thermostat and not the furnace. Remove R and W wires at the furnace and use a jumper to connect the two. Then turn on. Wait a few minutes then turn off. If the noise is gone that means the thermostat is causing the problem. If that does not cure the problem call the Bryant dealer that installed the unit or another bryant dealer because the parts can be expensive.