My guess is it needs rebiasing. That is a common problem with well-used class A zero-feedback designs.
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I never had this happen with my Aleph 3. I did have mechanical buzz from other components (Wadia 830 and JJ 828), and in both cases, it was the power transformer. I agree with Ghostrider. Whatever it is, will probably require factory servicing if you can't / don't want to live with it as is. If you are a good do-it-yourselfer, you may be able to replace it, but you would still need to contact the factory to buy the part. I have not heard anything bad about Pass Labs service department, and vaguely remember hearing good things.
Thanks for your responses. Its definitely a mechanical noise and not from the speakers. Someone told me that all US amps with serious power supplies buzz because of the 50Hz mains electricity, as they don't use toroidal transformers or other power supplies, preferring classic core transformers with iron plates because they are sonically better and offer fast response and resolution. I don't know if this is plausible, but I will get in touch with Pass Labs as suggested. Thanks again. I love this forum thing - never used it before!
Latest news from Pass labs is that the Aleph 5 deos use a toroid - made in the UK (so shouldn't be an electric current frequency problem). They suggest that with age and temperature, it could be a meltdown of the Sorbathane pad underneath the transformer. Apparently if pad
melts, it's a bit messy, but not a safety or performance issue. Guess I need to take a look inside!