Hi,
I used to get the rustling when I had the stock JJ tube in, but when I switched in a psvane 12au7 premium tube the rustling went away. Others have reported this too, and that it can be worse with nos tube stock.
It seems that it's something tube related. In terms of sound I find the JJ and psvane similar, so it's been a relief to get rid of the rustling at $50 a tube (they sell in sets of 2 only).
I can't give you any scientific reason why this happens, only my experience and that of others. I actually bought the psvane because of no-rustle reports and found it to be true for me too.
Cheers,
James