Once again, I am blown away by the ingenuity, expertise and sense of humour of our Maker Space membership.
I got excited last week when by chance I thought I had identified a strong contender for the source of the noise, but more about that in a moment…
To respond to your excellent suggestions:
@Gawain the contact mic is probably the right approach, and I suppose that was what I was really asking for without knowing the correct name for it. Those jez riley mics look like a good choice too. I have a pretty good pre-amp (I think) in both a mini mixer and my audio interface but have never tried them in a high-gain situation like this. Could be a problem, as the noise is very faint.
@techfolderkites has some great ideas also, especially frequency shifting to identify the culprit. Agreed that the noise is too quiet to move a big speaker in reverse, but it might be worth a try.
@Courty has a good point that this is almost more a seismographic event because the frequency is so low. I checked our boiler pump, as well as several other sources like the lift in our building, but none coincide with the noise.
@the_duckchild also has a good approach which I had not thought of. I have been meaning to buy a decent large diaphram condenser mic for a while, and this is a good excuse! Both mixer and AI have phantom power, so no problem with that. Looking at specs, some affordable mics go down to 20 Hz so might do the trick, which I did not know. I suppose that is why they come with shock mounts. I have used the phase reversal trick to cancel noise in television production before (shoot next to a fish processing plant) , sometimes it works sometimes it doesn’t. But worth a try to isolate the real character of the sound and at least determine the length of the cycle of on/off.
@emuboy’s contribution had me laughing out loud in a crowded train carriage to the surprise of fellow passengers! When I re-read my description of the sound, I was almost senseless and people were starting to move to the other end of the carriage… . made my day. Fortunately, none of them asked me what was so funny.
But on Friday, a breakthrough… or so I thought:
A tunnel boring machine! 2.5 miles away! Throbbing noise - sounds right. Intermittent - probably. Low frequency - definitely. And there was no noise on Friday night!! Cracked it, right?
Well, maybe not. The noise was there again last night (Sunday). Have they already moved the TBM and started work in a new location? Possibly. but I doubt it really.
So the quest goes on. Thank for all for your suggestions. Stay tuned…