Metal popping sound from fuel tank, after the engine has been shut off.

Spray some starter fluid into the throttle body. If it runs then check the fuel system. Fuel pump, relay, fuse. Make sure it has fuel in the tank.

The fuel pump is inside the fuel sending unit. The sending unit is mounted on the topside of the fuel tank. There is a large O-ring that seal the sending unit to the tank (so gas does not leak out). This is a tedious job and requires the tank to be dropped. if the tank is full, the task of dropping the tank will be more dangerous. The fuel filler hose, fuel vapor hose, fuel supply line, fuel return line, and the electrical harness will need to be disconnected before you release both tank support straps. ***NOTE: the two fuel-lines coming off the sending unit are well designed to make them easy to remove. Personaly this design is one of my favorites. No special tools are needed to release the connections.***After the tank is slowly lowered onto the floor then you can start removing the sending unit. There is a round metal ring with four securing tabs. You will see a light blue plastic pin. This pin must be pulled/removed before the metal ring can be removed. NOTE: make sure your work area is clean, free of trip hazards, and no sparks or open flames. After the metal ring is removed, then the sending unit can be pulled out and replaced.

Sounds like the catylitic, I wish I could spell, converter may be clogged or starting to clog. If you have the test equit. to test the exhaust output this will help, but since Idon't know anyone that has that type of thing just laying around, go to a muffler shop and see if they will assist you in solving your problem. They will usually check it out at n/c. Be Safe and HAGO!

sending unit in tank

It will depend on the size of the leak, although the engine management system may not be getting the "Fuel Pressure Good" signal so will not allow the engine to start. So change the valve, if only for safety's sake. You don't want fuel spraying around near hot metal.

if its coming from the gas tank area the only thing i can think of would be the fuel pump..they make a little humming sound, but if its making a howling sound it might be going bad. does this help?

I would look at a number of possibilities. The first would be to look for kinks or obstructions in the lines, especially at bends. Change the fuel filter. Check the electrical connections and wire harness for corrosion or bare wires. Repair whatever you find and use some die-electric grease on the connections or spray them with Rust Check. Use the green can.Make sure the ground at the tank is making good contact, shiny metal to shiny metal. Clean the inside of the tank and make sure nothing is left inside or falls in during re-installation. Change the rubber connecting line on the joints with steel lines. Sometimes the inside of the rubber line will collapse. Hope this helps.

This is how to replace it on a 1996 Blazer 4dr.

First, go buy the part you need at Rockauto. The link is for a 1996, but make sure you order it for your vehicle. Just do a search for a Window Actuator Rod Retainer. Rock Auto requires you order a pair of these, so make sure you order two. Each one has two of them in there so you will have 3 extras.
Window Actuator Rod Retainers - RockAuto.com

To open the window in the mean time while you are waiting for the part, do the following.
1. Remove the two screws on the door handle so you can pull back the door panel.
2. Pull back panel on top in the middle where it rises.
3. Reach in with a screw driver or your fingers to push down the lever. It's a metal piece on the left side against the wall to the inside of the blazer.
4. Open window and tailgate and remove screws on the sides of the tailgate. There is one on each side. Make sure it is the one fartest out and on the plastic panel.
5. Once the screws are removed, pull up on half the panel that is farthest out from the truck. Slide out away from the truck. The panel should come off with a little effort. Do not pull up on the panel closest to the truck, you will break the hooks that hold it in. I broke two on mine. The panel will slide away from the truck pretty easily.
6. Remove the screws holding the middle metal panel down. There are about 12 screws. They are the 9/32 socket screws.
7. Pull the metal panel up, first dislodge the metal poles connected to the two white clips on each side in the center of the panel.
8. You will now see your actuator with the broken blue clip.
9. Pop off the blue retainer from the actuator. This will take some effort but it will pop off. (here's where you can replace the actuator if it's not making a sound)
10. Place one end of the retainer into the loose metal piece that rotates (left arrow in third picture) and then pop the other end back on to the actuator.
11. Replace the metal panel and clip the poles back into the white clips.
12. Make sure your door handle rod is clipped into the lock before you close the tailgate. If not, you have to work at opening it again by sliding you arm down between the plastic panel and it's not easy.
13. Replace everything. Before you close the door, make sure the door handle works and make sure the actuator is connected.
14. You should now be able to use your tailgate window again!

I am taking a guess your trying to remove the tailgate panel to fix this problem???

Put the key into the ignition and turn it to the second position. This position will light up the dash lights but will not crank the engine.
Listen for a faint or soft humming sound coming from the rear of the vehicle, specifically the gas tank. This will happen almost immediately after you turn the key to the second position, so you'll have to listen closely and be ready for it. The humming is the sound of the fuel pump priming, or pressurizing, the fuel system. If you do not hear this sound, you may have a faulty fuel pump.

1. Check to make sure there is enough fuel in the gas tank. Although this may seem obvious, it happens to the best of us. If you're gas tank is near empty, the engine may not start. This happens more frequently in the winter time in cold climates; the low temperatures cause condensation to form on the inside of the gas tank. Since water doesn't burn, the fuel pump tries to draw in fuel and instead gets a mixture of water and gasoline and cannot get enough fuel to the engine for combustion.
   Check to see that the vehicle starts. Even if you don't hear the fuel pump priming, attempt to start the vehicle. If the engine won't turn over (start) and you haven't heard the fuel pump prime the system, it is a good indication that the fuel pump may have failed.
   
   Check the fuel pump fuse. If the fuse is blown, the fuel pump will not prime. Open the fuse box under the dash of your vehicle and locate the fuse for the fuel pump. Alternately, some vehicles locate the fuel pump fuse in a fuse box under the hood. Remove the fuse box cover (specific removal will vary according to the vehicle). Check the fuse to see if the metal strip inside the fuse has popped. Replace the fuse if the metal strip has popped; if it hasn't popped, it is most likely your fuel pump itself that has failed.
   

Unplug the turn signal relay from the fuse box and start you engine. Does it still make that noise?
If yes... Put the relay back in, that is not the problem.
If No...Then the relay could be bad, replace it!

PLEASE RATE THIS SOLUTION! Thank You Very Much!