Diagnosing Fender Blues Jr noise after diy mods

[ToneJaw Music]

After doing the supreme mod kit from frommel electronics, I’ve noticed some noise issues.
First off, the amp sounds great after plugging it up just for a bit, only noticed some slight low constant hum noise but not too noticeable. 
 

However, when I’ve got a cable plugged into the input Jack with no guitar attached, it makes this reasonably loud buzzing sound that starts up a few seconds after turning on. 
Not sure if this is by coincidence, but it began when I touched a multimeter to one of the power/output transformer leads when trying to measure bias. 
But, this buzzing sound goes away when guitar is plugged in, so I guess I could have worse problems. 
 

While doing the mods, one of the only things I was unsure of was replacing the input Jack from a plastic input soldered and clipped to the PCB and replaced it with a metal Switchcraft input jack that has isolated and shoulder washers. The instructions for this part were terrible and haven’t been able to find any sure info on if I wired and installed correctly, particularly when it comes to grounding. 
 

Can provide pics on how I did it if necessary  

Also, on a side note, does anyone have any exact instruction on how to measure bias in blues jr with multimeter?

Any help would be greatly appreciated

Thanks,

Jackson