Why is my vox VT15 buzzing?

I hown a vox VT15 amplifier, and whenever I turn up the volume to a little before half volume, it starts buzzing, and its really annoying, I play alot at loud volumes, usually around 2 o clock on the volume dial, anyone know what the problem is? And no smart *** answers of "go to guitar center" or "buy a new amp" thank you!