You threw the Nissan in there and are confusing us. Backfiring out of the exhaust had better be the Nissan. Otherwise you got a hell of a banging noise that is completely unrelated to backfire or popping that diesels don't do. If it is you have bad valves, blown turbo, aka serious problems. A check of the transmission oil, transfer case oil, rear end, and front diff oil is in order. A busted gear can bang. So can a broken or locked up u-joint, bad driveshaft center bearing, Broken flex plate, bad harmonic damper - you need to inspect the harmonic damper every oil change. I am driving around with a banging noise is a warning something is about to bang one last time say before locking up giving you a fun ride with a locked up wheel - this is to make you re-consider the serious problem the noise means with a diesel. We are not there so your description is all we have and it sounds bad. Do not drive it till you know what this noise is.
As it won't start. Hold up a second. Before you blow $100-200 in parts without a clue as to what is wrong consider a shop diagnosis for the same amount of cash. Otherwise you need to troubleshoot the damn thing before removing perfectly good glow plugs and the time chasing your tail - with no results and a lighter wallet.
To start the engine needs 100 RPM minimum to start. This is Not a gas engine that will fire hardly turning over. Does it sound like it's cranking slow? Then, yes, it's turning over slow and WILL NOT START, PERIOD. Weak batteries, bad battery cables, bad positive connector, corroded cables, bad grounds, 1 of 4 brushes in the starter failed cutting RPM in half. I have an optical tach to check cranking RPM. If you shoot a video of it cranking (30 sec max or the starter starts to melt down) and post it on here, via youtube, we can tell by ear if it's spinning fast enough.
Air in fuel is the #1 thing to start with. No fuel No-start. Air in fuel compresses and no fuel gets pumped. Clear return line and lift pump test is in order.
Electrical - quick and dirty. Leave key off for a full min (or the results of test are random/bad as the ECM doesn't fully power off.). Turn the key to "run" for the bulb check. Do you get both the Service engine soon and the service throttle soon lights? Yes the ECM has power. No, no ECM power = no start.
Glow plug light? Glow plug light is powered of the glow plug harness so if the lamp is on the relay is good and you have power to the plugs. A resistance test of the glow plugs gives you an idea if they are possibly good. Open is dead, some resistance is maybe. I use an IR temp gun to test them. Take and record reading of head near glow plugs on cold engine. Cycle the glow plugs several times. Measure by the plugs again - any temp rise means the plugs are working. You could remove an injector and verify you got a glow or IR temp gun a hot tip. GM says use an clamp on amp meter...
If you plug the engine in overnight with a working block heater it should start without a working glow plug system. We did this a lot in Colorado as our 1988 6.2 ate plugs and controllers... What a difference the internet and modern troubleshooting make for the glow plug systems.
Do the above without buying parts and you can at least get an idea of what is working.
PMD failure including harness is the only part we throw at 6.5's after we verify the above. PMD failure isn't clear cut sometimes. We will get to the loosen injector line to see if it's pumping fuel at all later.
