after first service engine develops internal knok in engine

We bought this new Frontier and I had Presidents day off and decided to take back to dealership for my Free first oil change at 2400 miles, after the service I drove back home and my wife noted that she could hear a knocking sound coming from engine. I took it back the following morning and the dealer kept the vehicle.They called me later in the day and said the engine would need replaced, and stated that the Nissan Rep had confirmed that the engine needed replaced, Very upset since we just made our first payment and were very excited to have our 2016 Frontier "Desert Runner" I requested the dealer exchange the truck and that we did not want a rebuilt engine in our just newly purchased vehicle. The service director stated that it was not their problem that the bank now owned the vehicle..we complained to Nissan Corp (case # 21737396) and they said there may be a opportunity for a "Buy-Back" that it would take 10 days for their review, 2-weeks later we went to the dealership to get our personal belongings out of the truck and the service manager said our truck was ready that they installed a new long block. They said to turn in the rental car and we were forced to take our truck back with the rebuilt motor. Upon the inspection there I noted disconnected ground wire, damaged firewall installation, and scratches.They reconnected the lose wire and ordered replacement parts for destroyed item. Once I got the truck home the next day I inspected under the truck and found that 2 of the transmission bolts had backed out , Nissan said they did all they could under warranty and that was their decision. We now have what we have coined a "Laundered Lemon", we feel that if they failed to tighten the transmission bolts, who knows what else is loose on the drive train, we feel that this vehicle is UNSAFE and have lost ALL confidence in Empire Nissan and Nissan Corporation and the vehicles they represent. Neither the Dealer nor the Nissan Corp would give us any information as to why the engine failed after only two months and 2400 miles. I have asked the dealer for an independent safety inspection and have not heard back from them as of this date.