Problem with tube amp

Over the last few days, my Cayin A88T tube amp is producing a soft thump from the left speaker. This happens erratically (after a few seconds and then after a few minutes)and sometimes it does not happen for an hour of playing. It is a midrange thump rather than a low frequecy one. I also get this now from the left channel on switching off the amp. The noise is produced even when the volume is set to zero and the CDP is off. Reversing the speaker cables shifts the noise to the right speaker.

Other than this noise, the amp sounds clear without any distortion when music is played.

I have got the amp checked out by a technician but unfortunately the noise probably did a dissapearing act when he was checking it. According to the technician, it could be a problem in the power stage instead of the prestage as the noise is heard even when the volume is at zero. He also checked and compared the voltages of the left channel with the right one and they were pretty same. Therefore, according to him the components are fine but I boubt it as some part is probably failing.

Any clues as to where the problem lies?