If it is when you turn the amp off and then back on...  it is probably being caused by the OA3 tube, which is normal behavior for those tubes, however it is easy enough to figure out if it is that tube or something else by swapping tubes on the left channel with tubes on the right channel to see if the problem moves to the other side and if it does, then you can narrow it down from there by changing only one tube at time.

Steve

checking the power tubes first. The Torii Jr V2 uses two 6L6GC or 6V6GT power tubes, one for each channel.