The loud "click" from the firewall is the battery isolater. It is a relay that opens and closes as it senses voltage of the two batteries, the vehicle (main) starting battery and the house battery bank. I talked to Roadtrek and to the battery isolater tech service guy at SurePower and concluded that this is normal under certain circumstances. (Took a while to noodle this one out, as, of course, neither party completely understands the issue, regardless of what they say.) Namely, if your house and main batteries are discharged such that the total circuit voltage is below 12.8, the relay opens to charge only the main battery. When the main battery reaches 13.2 V or more, the relay closes to charge both sets of batteries. If your house batteries are really low, then the total circuit voltage will quickly drop below 12.8, whereupon the relay opens to protect and charge the main again. When it reaches 13.2, the cycle repeats. There is a time delay built into the electronic circuitry of the isolater to prevent chattering of the relay. So this cycle repeats over and over, and each time the relay closes the house battery gets a chance to charge, and it comes up a bit, therefore taking longer to draw down the total voltage. At first, the cycle interval is short, but gradually increasing in time until it stops completely when the house batteries reach 13.2 V. Then it continues to charge both sets. Maybe this analogy will help. It's kind of like if you were to put a fresh cell and a discharged cell in a flashlight. The dead one will drag down the total voltage and give a dim light.
I've experienced the click-clank sounds on several occasions, mostly after we are dry camping and using the house batteries for a few days without recharging. When we start driving again, the clicking goes on for about 30-45 minutes, then stops. Scary noise until you understand what's going on. Then it's reassuring, letting you know that your batteries are being charged.
So I would say there is nothing wrong with your unit, the noise is normal. In the case of gkim, if you have undervoltage because of bad batteries, the isolater will continually cycle. I too have replaced my OEM house bank with AGM's, but for a different reason. Be sure you switch the inverter/charger to the charge protocol for AGM batteries, they act differently than wet cells. See your Tripp-Lite owners manual for details.