The usual mechanism for a chattering contactor is-
It closes, connecting the heater
The load of the heater makes the voltage drop
The lower voltage releases the contactor
The voltage rises back high enough to pull the contactor in
Repeat until failure
The resistance of a heater is lower when cold, meaning that it will draw more current until it heats up. While chattering, the heater is probably getting enough current to eventually increase its resistance, lightening the load, until the contactor will stay in.
First look for poor connections. See if the plug gets warm or if any of the terminals in the unit get hot (after running with the unit unplugged, of course). Anything on the magnet poles can aggravate the problem, but that will also usually make the contactor buzz.
Bill