It's most likely caused by dissimilar metals expanding at different rates. In the past I've owned three Pass amps from the .5 and .8 series and they've all done that to some extent. After about 45 minutes of warmup it should subside.
I spoke to an engineer at the Pass factory anout the problem and they advised me to loosen the bolts on the top of the amp. It didn't completely fix the problem but it reduced the number of klunks. Mine would make 4 or 5 loud klunks during the first 30-45 minutes of warmup. Good luck.
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