Aleph 5 - buzzing sound when hot?

I just acquired and Aleph 5, but I have noticed that as the amp gets progressively hotter (after say 3 to 4 hours) of use, the amp start to make a buziing sound, which gets progressively louder with time. Is this normal for Aleph amps and if not, any ideas what it could be \and what I should do about it? The sound is definitely from the amp and not through the speakers.
My guess is it needs rebiasing. That is a common problem with well-used class A zero-feedback designs.
Send a note to Pass and ask (check the Pass website for contact info). I don't think the Aleph series has adjustable biasing. Is this a mechanical buzz or some thing you hear through the speakers?
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!
Apparently the Aleph does use a torroidal transformer, so this is unlikely to be the cause, although the bit about the 50Hz electricity can be true. Anyway I have contacted Pass Labs. When I get a response, I will post the answer here.
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!