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?
I don't know about biasing your amp,but if you could try swapping tubes between the left and right also.It could be a cap starting to leak,or other small part failing.Most likely
something small,tube first choice,then cap etc.
I would look for another technician.A good tech would have stayed with the problem until it was solved,no matter how difficult or easy it was to find and/or fix.Dedicated and competent crafts/repair persons are getting harder and harder to find,especially when it comes to tubed equipment.
I have the 88, and have not had the problem, yet. Checking the bias and tubes sounds like next steps, and should have been done with the tech who you used.

Before you spend and more time and money, best to call on Steve, at Steve is the distributor in the US for Cayin and is also a very talented tech. I've seen his work bench and it has all the tools that a great tech should have.

Tpreaves makes a great point about finding techs who know tube gear. Best to go to the manufactuerer/disbributor from the start. In the long run it is less expensive and gets the problem solved the first time.
Intermittent noises can sometimes be a bad power tube. Hae you tested them?

BTW I have this amp and I have not had this problem, but I was having intermitent noise issues with my Onix SP-3 and it was a bad Valve Art 350B tube.
Thanks all of you.

I swapped the tubes from one channel to the other and the noise shifted to the right channel. So it is a tube problem. I will have to get new tubes.

I was guided by the distributer of the amp to the technician. In my city (Mumbai), there are very few technicians one can trust.

Is the amp schematic necessary to repair an amp? I have written to Spark Audio for the shematics but they have not answered my mail as yet. Do you know where I can get the schematics? Do manufacturers generally share schematics?
A good company should sell you a schematic.They probably sold
one to the technician they sent you to. I think some technicians could fix stuff without one,but they need one for the specifications to make it exactly right.