Tenant with lots of noise and vibration

[Atul Mohlajee]

I have a strange problem.  I own a 6 unit apartment building.  On first floor, there is a 50 plus lady who moved in alone.  Tenant in basement apartment (legal unit) complained of unusually high noise and unusually high vibration activities some nights. Her next door neighbor also complained.  I and the basement tenant are worried about ceiling coming down.  How do I put it nicely and politely to her without creating uncomfortable atmosphere for her and me?  She has a lease for herself alone but another person might be living with her (too early to be sure, she only moved in two weeks ago).