Troubleshooting an Engineer's Thumb build

[DIY Dood]

Just built an Engineer's Thumb using the kit from musikding.de. Here's the schematic for their 5 knob implementation:
https://www.musikding.de/docs/musikding/engineer/engineer_schalt.pdf

It's currently outside an enclosure for rough testing. And it seems to be roughly working. I get some odd effects with the controls like level and ratio up high... but I think I can ignore those for the moment. Obviously there's lots of hum with no box to shield it. But I have an issue that I think is beyond that.

Specifically, when I touch the case or shaft of the Threshhold pot I get serious hum. It sounds like I'm touching the input jack hot side. If there's sound running through (Ex. strum a chord), it sounds like a staccato flanger when I tough the control. With the switch in bypass mode, sound is clean and clear as expected.

The shell is not touching any circuit component outside the three soldered leads. I took it  off the board and tested it to see if there was any leakage from the pot terminals to the case/shaft. There was none even down on the single ohm scale. Reinstalling it after testing gave me back the issue.

No other pot has this issue.

If I ground the pot case, the issue seems to abate. But it doesn't seem like the right solution.

Thoughts?

EDIT: I'm a newb... if you want me to do things like check voltages you have to be fairly specific.

[blackieNYC]

When you put in a metal box it will be grounded. Hopefully.

[PRR]

Yes, ground the pot bushing/case. The Thresh pot literally goes-to "the input jack hot side", and is the highest impedance point in the project, so is very little different from just poking the jack with your finger.

It is remotely possible something has come loose inside the pot, but I think it just wants to be grounded.

[DIY Dood]

So you think it's just a lack of shielding (no case) and the case/shaft of the pot is doing something inductive?  Again, the pot checks out OK and the case isn't grounded to the circuit. I'm thinking that touching it should have no effect.

[PRR]

Maybe I'm wrong. Try it both ways.

[DIY Dood]

So, in a box it's better, but it clearly has issues. It's much quieter, but touching the metal box cuts out some noise it's throwing... I'm guessing that's a grounding issue?

More importantly. It seems to be doing some compression, but the whole thing is very crude. Threshhold seems to have limited effect except to push it to dull-mud at fully CW. Attack and Release have no perceptible effect.  Ratio seems to work and blend clean vs "compressed", but fully CW is very harsh. Gain also seems to be working, but at close to fully CW it's breaking up badly. No one would consider the sounds I'm getting usable. 

How do I go about checking things out. I looked for the obvious... bad joints, etc. but I'm a newb. I'm sure I've messed up some things. I need some diagnostics to use to find troublesome areas/problems.

Guide me if you can,
Thanks.

[DIY Dood]

Closing this out... I realized that specific circuit questions are better posted in the year's long Engineer's Thumb thread, so I've moved to there with my latest data/questions. Thx.