Hey everyone! So lately I've run into an issue with my Matchless Cobra combo. Whenever a signal is present in only the 2nd input (low gain), the amp makes a static-like crackling noise. My intuition was that this might be an issue with the input jack, but everything checks out okay. Any ideas? Video link below!

There's not really anything it could be except the input jack. There's all of one resistor separating the two inputs before they merge, so anything beyond that would cause the problem on both inputs. I'd be shocked if that resistor were the problem, they're not exactly known for causing this type of issue.

Not sure what you mean by "everything checks out okay" but I would bet that a proper cleaning of the jack (all inner surfaces with contact cleaner, re-tensioning if needed) and re-flowing of the solder joints, or just replacing the jack, will fix the issue.