My first impression was that a cat got under my bed and it was shaking it, but then I realised that it's impossible as the bed is way too heavy. (Plus there was no cat, I checked!) 
Kwa mtazamo wangu wa kwanza nilidhani ni paka aliyeingia kwenye uvungu wa kitanda na alikuwa akikisukuma-sukuma kitanda, lakini nikagundua kuwa hilo lisingewezekana kwani kitanda ni kizito mno. (aidha hapakuwa na paka, nilihakikisha!)