Help: Noise in Left Channel

[M100C]

I purchased a Crack amp which "needed work".  It has the Speedball upgrade, and BH power cord.

Rolled the tubes, and there were some cold solders, but it sounds gorgeous now, but has a pesky problem.

While powered up, the left channel exhibits a low hum which comes and goes in my Senn HD650s.  Right channel is clean.  It is not volume related; without input, the hum is the same at no volume or full volume.  There is NO volume-scaled noise.  I have rolled both tubes; no difference.  About the noise -

It sounds like 60Hz, and it is best described as sounding like a Star Wars light sabre!  If I gently tap on the chassis top, wood enclosure, tubes, etc ... it is microphonic coming thru the left channel.  It sounds just like two SW light sabres coming into contact.  Sometimes, light pressure on the volume knob (upgraded Alps unit) helps.  If fact, if I tap the wood enclosure, it will diminish (release the noise, sometimes fully), but it will build back,  It is very low volume, and is easily overcome, so it doesnt really affect listening, but the right channel is perfect.

Thoughts before I start tracing?  Where would you start?

Chris

[Paul Birkeland]

I would reflow all of the solder joints, take the Speedball out and install the loading resistors, and remove the Alps pot and put the stock pot back in (unless the Alps pot is mounted to a PC board and the wires are soldered to the board).  It also helps to do voltage measurements which are listed in the manuals.

These recommendations come from fixing dozens of Crack amps over the years. 

The light saber sound is somewhat reminiscent of what I hear when non electronics flux is used in our kits, what were the initial build issues?