Air leak even when not under load

Hi there, I'm having some trouble with a diesel compressor and I see you know your stuff. Its an Atlas Copco XAS67 with a twin screw pump. I have rebuilt the engine for my customer and run it in for a few hours with no issues. I then took it in to run it up under load, but when I opened the air valve the engine revs increased as they should and it gave air, but there was what I can only describe as the sound of something bursting, and now there is what sounds like a continuous air leak from somewhere around the compressor, even when its not under load. I cannot find a leak anywhere though! I wouldn't worry, but it wasn't doing it before, so there's obviously something amiss! Many thanks in advance. Mark Kearney