Left Channel Producing looping heartbeat clicking sound like a clock

[gabu840]

Hello Everyone,
My amplifier(Sex2.0 with C4S Upgrade) is producing Pumping Heart-Beat sound from left channel in a continuous loop, also sound is coming but staggered.
Please suggest what things do I need to check, could it be :
1) Ground Issue
2) Supply Issue
3) Capacitor Issue

Please support to pin point exact root cause of this problem. I have checked the tubes via Interchanging them, both are working fine flawlessly.

 

[Paul Birkeland]

How are your voltage checks?

 

[Doc B]

Look over your capacitors to see if one of them may have been nicked by the soldering iron.