“The Hum” sounds different in different areas. If the signature is the same everywhere that’s quiet enough, its probably you. You could get a hearing test in a sound proof booth, they’re pretty quiet, and if you hear it there call a priest.
To measure in the field, a spectrum analyzer app. This is probably the best one ( I used to be an audio designer) but with its flexibility comes some opportunity to screw up the settings https://apps.apple.com/app/id490078884
This one is simpler but has less resolution and less knobs to twiddle which may be necessary to catch a low level and low frequency sound: https://apps.apple.com/app/id355396114
I use them both, they’re good
Make sure your mic has low frequency response to 20Hz, lower if possible. Some mics roll off the low frequencies outside the vocal range (~ 60-80 Hz), to reduce background noise, but that’s what you want to catch.
Good luck! LMK if you want me to put in a word in at my parish ;)