Not in my case it wasn't.Anything to do with saddle, seatpost or pedals would consistently be at the same point in the pedal stroke.
+1 for seat,
I had a noise with similar symptoms for ages until I realised the tension adjusting nut on my Brooks B17 needed tightening slightly and silence was restored...
You should start a '24 Carat plonker' thread.See the " I am a 42 Karat Plonker " thread, in the cafe.