Guys, good morning and thank you all for the input.
Do I dropped by my LBS after doing a 80km which by then the sound audible all the way - back pedal, front pedal etc. The mech dropped the wheel, and whoala, once hearing the sound he knew what it was. Turns out it was the back wheel hub with gaskets that had gone dry. the hub was greased and oiled and now the unit is as silent as ever. lesson learnt for me as well - rubber sqealing that's non-metallic sounding in nature : possibly wheel free hub needs lubing. cheers.
this was driving me nuts and I'm glad that its all resolved now.
to answer the gent above - yes, DT Swiss 540 rear hub.