Thanks Paxboy, I appreciate the response. I initially thought it was something to do with the overhead cabins as well, until I noticed that the sound was occurring during perfectly calm stretches of the ascent. It also ceased entirely (apart from a few odd groans here and there) once we reached cruise altitude, and was not noticeable during the descent in spite of moderate turbulence.
I've also read that metal fatigue has been a serious concern on Boeing 737s (admittedly the older models), and less stringent safety / maintenance protocols here in Oman might lead to such cracks slipping through the proverbial one.