Sounds like a loose wire. As much as I love my ASL headphone amp I think the quality control is suspect. Check the soldered connections inside. You might find a loose solder or wire.
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I have one of these and never had a single issue. Does the amp produce any audible noise directly from the unit or is the noise just heard through headphones?
Also, have you hooked up the amp to run directly into a power amp and through your speakers? If so, did you experience the same noise through the speakers that you are experiencing through your headphones?
The unit I have is the OTL version and can be switched to play OTL or with the transformer. I point this out simply for the possibility that it could be the transpormer (in addition to a loose wire as stated above), and if your's has the OTL option to try that. I obviously don't know details about their different amps, hence the above comments on the OTL aspects.
I have had great success with mine and the tube rolling is very productive.