In one channel or both (from your post it sounds like one channel)? If in one channel swap the tubes from left side to right side. See if noise changes channel. If it does just replace the offending tube (which you identify by then switching only one tube to the opposite channel. If the noise follows the changed tube you have the culprit.
Noise problem as tube amp warms up
I recently installed some NOS Russian 6922 tubes in my Sonic Frontiers Power 3 monoblocks. One amp makes a siren sound ascending in pitch from low to high as it warms up. After that it seems to be fine. It sounds a lot like an analog synthesizer from back in the day. Can anyone help me diagnose the problem please? Some preliminary research suggests a bad input tube tube or possibly a filter cap. The old tubes made a similar sound, but softer and descending pitch when powering down.Thanks.