Regular low frequency click with A15MK2 deluxe even when volume at 0

Dear All,
I just finsished my Madamp A15MK2 deluxe and both channels work fine.
There is no hum but I noticed a low noise low frequency permanent click (arournd 2 to 3hz) as soon as the amp is ready to play.
I suspected a problem with the power board but all the measures are right and it is not a 50Hz hum. Problem with the output transformer?
I inspected the past threads but I did not found anything about this problem.
Any idea where the problem can come from?
Thanks.
Jacques

Thank you Martin for all your suggestions!
The noise is regular and it is not a popping noise nor crushing one. But the noise is slowly evolving to clik+hum as the tubes go heater (10 min).
I have the right tubes in stock to make tryings in first on them.
Thanks too for your idea to try the "send" output to identify the noisy stage.
I will make systematic tryings during the next weekend and I keep you in touch.
Kind regards.
Jacques

Dear all,
Thanks to Martin's council I found the origin of the issue. I post it in order to eventualy help other beginer forum members like me.
I apologize if my explanation bellow are too elementar for the expert.
After some tryings it appears when I connected the A15 send to another power unit, the regular noise was a lot louder with this power unit (plus, this time, little pops and crashing).
After that, connecting the A15 return (or send) with a cable (open or not) make the noise instantanely stop and the A15 absolutely silent.
So I deducted the power stage was not at the origin of the issue.This also indicates the power supply and its filters were not in cause.
But the preamp stage (both clean and boost channel) was not obviuosly responsible since the noise was present even with the volume at 0 or at its maximum.
So my attention focused on V3 or V4.
I tried to change V4: no change.
But V3 was the bad guy!
All works fine and silent now.
Jacques