Some tube equipment companies’, "Standby" practices, keep the gear at half voltage and zero B+. That avoids thermal shock, at turn on. Sonic Frontiers doesn’t mention the half voltage thing. Still; keeping the filaments energized, is supposed to better for the tubes(etc), than a cold start. Not to mention: warm up, as regards presentation. You may have a tube, with an internal glitch, which corrects after fully heating. If the noise only occurs in one channel, try swapping the tubes between channels, one pair at a time, to locate the offender. If in both channels; there’s probably another issue. I’ve always given my tube gear a few minutes(each piece) to stabilize, before bringing anything else online.