Sources of very low tones are many. Keep in mind that there are two parts, the tone generator and the speaker. The tone generators for low tones could be engine associated pumps, gears, shafts etc. Unlikely internal gas path stuff.
The speaker is more likely to be the nacelle ,engine ducts. Very very unlikely to cause damage although upsetting to passengers.
There are some tones that could be of concern but always associated with major vibration signatures (things visibly shaking).