Ian, since nobody but you has heard this noise, what kind of "loud noise" is it? Where does it seem to be emitted from? The drivetrain in particular? If it is a "creaking" noise it is most likely ungreased metal to metal like on pedal threads, pressed in cups. stem/steer tube, seat related or missing washers or adjustment screws.
Worst scenario is a cracked crank arm or frame ready to let go. Have you "felt" for the noise while riding yet?
Squeaks can be located by feel/touch through vibrations sent through the frame. 'Do this while riding...You can also get a can of WD40 and start blasting everything until it disappears while never locating where it was..