Throwing some random ideas:
Shoelace could be hitting the crank (not very likely, but have had that happen - hollow frame tubes are extraordinarily good at transferring sound).
Bolts on the rear side of the frame might be producing the sound. Loosening those, putting some grease (or, better, some
anti-seize), then re-tightening them could do the trick. This is more likely if the bike has a rear rack mounted, otherwise not so much.
Finally - as some have mentioned, saddle clamp (if it can be removed), and seatpost, as well as saddle to seatpost interface and bolts could all do with some paste (for carbon frames use the carbon mounting paste, for metals use anti seize, or grease).
My
method of finding and eliminating strange bicycle sounds.
More general (generic), but tried to put it as methodically and systematically as possible.