Gas devices typically have a flame sensor. The typical sequence is:
- call for heat
- if there's an exhaust fan, turn that on to purge any combustible gasses
- open gas valve and turn on ignitor (might be simultaneous, or a slight delay between)
- sense if the burner lit
- if not, purge the burner of combustible gasses
- try again. If after (depends on the system) X tries, and no flames sensed, lockout the system
Your system may slightly differ, but that's the general idea.
So, if you hear things trying to turn on (it may have started to make those sounds if you shut off power to it, resetting the lockout), and it can't sense the flame, it will eventually lock out again.
The flame sensor can vary depending on the system. It could be a microcurrent probe that is shot, dirty, or bent so it's not directly in the flame. Some use a hot surface ignitor, and that can also act as the flame sensor. WHen it's trying to light, are there clicks, or just that hum? If there are clicks, it's probably a piezoelectric sparker. If it just hums, it's probably a hot surface ignitor. All of this would be discussed if you found the operator/installation manual. FWIW, sometimes, they stuff it inside the unit. Have you looked? THey usually often glue on a copy of the functional diagram inside the door. IF you find that, take a picture and post it.