2007 Ssangyong Rexton engine chirping

A chirping sound has slowly developed in the engine and is now quite annoying. The fanbelt and tensioner has been changed. Its frequency seems to be directly the same as turn over speed and disappears above a certain rev speed, typically about 50 miles an hour in fifth gear. On idle the chirping sound almost has a knocking element to it. I have heard other rextons standing in traffic and also a Rodeos or two with this exact same sound coming from them. My very able mechanic is scratching his head on this one. The car has visited the dealership where it was bought but they have been very silent. I was suppose to be contacted back but never was. Anybody have any experience which might help me please.