Touch cap top quiets noise?

[Axtman]

Working on a GK 200MK amp. The amps hums a bit. I went around touching the radial electrolytic capacitors to see if maybe some of them are loose. I found one cap in particular that quiets the hum when I touch the top. Why is that? Is that cap possibly soldered in backward? (I would have to go hunting around to see where that cap is in the circuit. That amp has a LOT of electrolytic caps!)

[The Dude]

Is the cap well soldered into the board? Maybe pushing on the cap is making a connection.

[Enzo]

I have a different theory. When you touch it, you are injecting some hum. This new hum happens to be out of phase with the existing hum, and so it cancels out the hum.

[Helmholtz]

Does the hum also get lower when you touch the chassis/amp ground?

[pdf64]

Electrolytic cap metal bodies aren’t necessarily at 0V common. With those used for the negative power supplies etc, touching the cap metal may result in a nasty surprise.

[Mick Bailey]

Most small radial electrolytics have no direct connection to the can, unlike axial caps where the can is usually negative. I have noticed though that there can be a voltage on the can of radial caps that is often unrelated to the terminal voltage. Maybe there's enough resistance to show a voltage leakage that's pulled down by the DMM's own resistance. Large radial caps, especially older types, can have the can connected to negative and I have had the occasional jolt off a can on the negative rail.

I'm thinking that in the case of an unconnected can, touching it grounds it and creates a screen. Just another theory.