I agree, it sounds like some mechanical or electronic device or almost like a drum program on a phone or something. If it’s always the same rhythm maybe a contractor dropped a phone into the space? Does it really stop and start or is it just more audible at certain times of the day?

Could it be a attic space exhaust fan, some are powered. Maybe the roofers disturbed/bent fan and it's not running well. It sounds too rhythmic to be animal, but not sure.

I agree on the rhythm

there is no resident above or to the sides of the unit, those units are used only seasonally

so isn’t any human variable if clocks or drums

but given the disturbance started after roofers were here I do think it could be from some fan as suggested

To the question in if it really stops and starts

it fades in/out even when standing in the same place listening

and where the sound comes from in a more pronouncedplace , like sometimes clearer in kitchen wall va bathroom ceiling

I can’t seem to hear it running non-stop so can’t figure out the trigger

I can hear it most above a kitchen and also a small bathroom,

Possibly some exterior noise nearby that is traveling down plumbing vents? More audible at night when noise level is lower? Variance due to wind/ wind direction? Bathroom vent somewhere in the building w/ automatic humidistat?

Doesnt sound like critters to me. It does sound like its a clock or a radio that is set to come on at a certain time. You may be hearing it more because of the hole in the wall, you mentioned, or has that been closed in. The last occupant, might have set something to come on at a certain time, and it was never noticed, so wasnt shut off?

Contact the absent neighbor to gain access to his empty condo... likely he'll be interested in the issue.

Yep already on it. Permission to enter and waiting for prop mgr to bring key For second visit. He was here yesterday and we couldn’t find anything. asked me to observe . Heard it all night again and thought I’d get ideas from here!! Might be the vents with something trapped. Also going to look for any old clocks upstairs.

on the suggestion of hearing it now bc of the wall, I like your thinking wondered that too, but the hole in the cabinet repair has been there for weeks also the noise is in another place too not near the hole. The timing of this synced with roof repairs . I think it might be something in a vent....but what?? And how to get it out ???

it’s a mystery!

I had a job involving noise reduction in my youth. I actually wore a Stethoscope for a couple of years to better hear unknown sources. Get a cheapo - either medical or automotive sources.

$5 Cheapo Stethoscope

PS The mechanism of sound transmission in structures can be very difficult to understand. I "chased" a rattle for nearly three years before it was localized - and it was not even near where I had tentatively concluded it was!