My water heater makes a loud boom sometimes when it starts up. It has a electric ignitor.

Well its not right should not lighting with a bang or boom Its not lighting on the first stage pilot only and it sounds like you are getting full gas . In 10 visits I would ask them to call B gas or Baxi to fix this as this guy cannot .

My guess is the ignitor needs replacing (old and has built up too much internal resistance). Even though it still works at times, I would suspect it first, even if it glows during an ignition cycle.

Make sure the electricity is turned on, most new units have an electrical ignitor which must be turned on and heat up before it will allow the gas to come on.

check the gas pipe carefully spiders like to make home in there and clog it

Identify type of water heater:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
http://waterheatertimer.org/How-to-fix-water-heater.html

Gene
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Probably heat trap nipples, or other constriction in pipes.
http://waterheatertimer.org/New-water-heater-making-noise.html
http://www.thetankatwaterheaterrescue.com/forums/forum3/
Add a comment and say what you find

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Replace heat trap nipples or inserts?
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#heat-trap
Factory Heat traps save $2-$9 per year but can cause noise, and you can make your own heat traps.
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#heat-trap
Electric water heater is probably the only household appliance made today that can be maintained and repaired by homeowner to last almost indefinitely. Certainly no other water heater can last as long, even with maintenance.
http://waterheatertimer.org/How-to-maintain-water-heater.html
Add a comment and say what you find

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Copy following link for troubleshoot by gas control valve.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

inspect and clean arrestor screen
http://waterheatertimer.org/images/Flame-trap-inspection-325.jpg

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

I would check the metal door on the bottom near the burner, sometimes they come loose and vibrate. If not that then I would suspect the chimney liner inside the tank, I have seen them corrode and start vibrating when the gas is running

The red button you pushed on the hot water heater was probably what is called a "piezo-electric" ignitor. It's normal for it to make a snapping sound and then pop back out. It sounds like you just lost the pilot light.

That being the case, do this:

Turn the gas valve on the hot water tank to the "off" position - it should be silver in color and down low on the tank. You may have to push it down to do this
Leave it off for about two minutes or so
Rotate the valve back on to the "Pilot" position
Hold down the pilot gas valve - it should be red and immediately to the left of the main gas valve.

While holding the pilot gas valve down, push the other red button to the right of the main gas valve - this is the piezo ignitor - push it several times in a row quickly

The gas pilot light should light
You can stop pushing the ignitor now, but continue to hold down the gas pilot valve (the one just left of the main gas valve). Let this valve up after about another 20 seconds or so.

Now rotate the main gas valve to the "on" position

Listen carefully - you should hear the main gas light and feel heat at the water heater flue outlet on top - don't touch it with your hand - you could get burned

If no luck, repeat the process until the heater re-lights

TIP - the instructions for re-lighting the pilot should be written on the heater

Just a potential diagnosis - the thunder clap, if as close as you describe, may have been intense enough to create what is known as a "draft inversion" that may have snuffed out your pilot light. The air pulse came down the flue pipe.