Need your expertise/investigative skills re a vibration in building

[securespark]

A battery-operated "massage" device?

 

[ladylola]

That's some "massage" device to shake the bed in the flat above. Mains powered industrial jobbie rather than batteries more like.

 

[Doug99]

Use a long metal rod as a 'sound bar', by placing one end on the floor and the other to your ear (eg long screwdriver) By changing locations you should be able to identify where it is most noticeable/louder and then be able to equate it to a position downstairs for investigation. Mechanical engineers have used this method for eons to identify source of problems in engines and such like.