Just checked that clip and it's definitely not that...just gone back to the comp...the tone is at about 16-17khz when audio is being passed. When the audio stops a low and noisy oscillating sound creeps in. Bloody annoying....one week of silent operation them this happens!!!
DISCLAIMER: i think i get this circuit, but somebody should jump in and hit me with a rolled up newspaper if i say something stupid here.
i'd still do a tap test to rule out microphonics, and possibly number and swap them. if there's a bad tube at V1, the introduced noise will be amplified by the 'good' gain stages, so whichever configuration gives the most noise should dictate swapping of V1. if low-freq problems are still persistent, i'd check your grounding of the grid leak R's (13, 14, 21, 22). if these aren't grounded properly, your filtering from the RC filter formed w/ the coupling caps (c2-c5) will be compromised.