Amp Makes Thud When Switched On [resolved]

[RdDgWest]

The left amp when powering on makes a low level thud in the left speaker when turning on.  I did not notice this the first time I powered on the amps after the build.  I do not hear the thud when turning on the right amp.

Besides the low level thud everything sounds great and appears to my hears to be working.

Is there something I should be looking for on the circuit side of the left amp?

[Paul Birkeland]

I would be more surprised that the other amp doesn't make more of a thud.  It's possible that if you swap 2A3s that this could follow a tube, as warmup behavior will be somewhat influenced by 2A3 operation, but this isn't something I'd lose sleep over.

[RdDgWest]

I was going to swap the tubes between channels one at a time but wanted to mention this first. 
Let me try this this evening.
Thank you, Paul

[RdDgWest]

I swapped both tubes one at a time between amplifiers.  It appears to stay with the amplifier and does not follow the tubes. 

Still not a worry about it situation? 

[Paul Birkeland]

You might want to check over the solder joints of the amp that doesn't thump rather than the one that does thump.  Particularly the wires going to 5/10 of the output transformer, as well as all the 4 pin socket wiring.

[RdDgWest]

I listened all week without an audible issues. 

Upon visual inspection I felt the solder connections looked good but this morning I re-soldered the 4-pin sockets, connection out from the 4-pin sockets, and both ends of the wires that connect to pins 5 & 10 of the output transformer.

Now I hear a thump in the speaker connected to the right amp. 

Thank you Paul