Which word fits more sound or noise in this sentence?
Suddenly, I heard an explosion sound/noise.
My suggestion: Neither. The phrase
'Suddenly, I heard an explosion'
is enough, there is no need to identify the actual sound separately, it's already indicated by 'explosion'.
Without the context of knowing that it actually is an explosion, I would suggest
Suddenly, I heard what sounded like an explosion.
although other answers may be more appropriate if you can identify the explosion from more than the sound alone.
Explosion sound and explosion noise both sound wrong to my native-speaker ear. I would only ever say "I heard an explosion" or "I heard the [sound/noise] of an explosion".
I can't quite figure out why this sounds wrong, though. You certainly can use "sound" as part of a compound noun, for instance "flute sound" would be perfectly fine ("flute music" would be more usual, but only because musical instruments are very strongly associated with, er, music). It might be because explosion is an event rather than an object ("crash sound" also sounds wrong).
As an alternative as @MeanGreen's answer, I'd say:
Suddenly, I heard the sound of an explosion.
This ^ is the best alt in my opinion.
I'd agree with the above suggestion of, "suddenly, I heard an explosion".
As I have a degree in sound, I thought it would be worth mentioning that, in acoustics, there is a difference between sound and noise. Noise is an undesired sound or signal, for example electronic interference. In the context of the environment that this explosion occurred, it might be considered a noise, as it probably wasn't a desired sound in that setting.
If you're set on including "sound" or "noise" in the sentence, the adjective form of "explosion" can be used to describe the sound as "explosive".
Heard an explosive sound
Heard an explosive noise
The phrase "heard an explosion" is likely better, though, as noted in other answers.
N.b. "explosive sound" may be slight agrammatical for what you're trying to convey. An "explosive sound" would be a a loud and violent sound, but the source of the sound is left unstated. An "explosion sound" or "explosion's sound" would be the sound resulting from an explosion, even if that sound is quiet or otherwise non-explosive.