It is not common with the 3-4 DNA-1 amps I've used. I suggest you consult with SMC about it. They are pretty straight.
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Do you have a 0.5 with a revision c, b, or a? I was just talking with somebody about this. It appears that this sometimes frightening thump has to do with wether or not your particular model has (I believe them to be) input relays.
If you have a Revision of the DNA 0.5, then if that model had the relays and the relays were removed, then the thump appears at startup and/or shutdown.
You should first contact the previous owner to see if he had anybody work on the amp.
Next contact Steve Mc smcaudio.com providing him with the serial number and model to see what he has on file for this particular unit. And of course describe the problem you are experiencing.
I have had a mixed bag with the McCormack's I've used. Most of them didn't thump. But I did have one that did. As mentioned above, it may have something to do with the mute relay. Seems the older they get the more likely this is to happen.
Call McCormack and ask. He's a really good guy and shoots straight.