You are mildly electrocuting yourself, and your muscles are contracting at 50Hz (Europe) or 60Hz (Americas) similar to how a TENS unit is designed to stimulate (see Transcutaneous electrical nerve stimulation - Wikipedia)

The electrical current is low-enough to not hurt / burn you, but you’re on the edge of danger… if you were a better conductor, that leakage current could be enough to stop your heart, especially if the ground-path was through your chest.

I have trained my hands to gently touch metal surfaces that I have any concern for stray voltage, versus grabbing hold firmly. This light-touching allows the outer layers of (dead) skin to act as a high-resistance, limiting the current. For instance, when I walk by a light-pole at the street, I check for that “vibration” before leaning on it directly. You’d be surprised how many people (and animals) are killed yearly from making hard-connection between stray voltage on metal and ground (ie: https://www.washingtonpost.com/local/public-safety/officials-reviewing-faulty-wiring-on-mgm-handrail-where-child-was-electrocuted-according-to-early-investigation/2018/09/25/54c00a1c-c029-11e8-90c9-23f963eea204_story.html?noredirect=on).

Be thankful you aren’t well-grounded, sweaty, or the voltage in that circuit isn’t higher, else you’d be the next statistic. Fix or unplug anything you touch that gives you the “vibration”.