Fish vibrating when touched

So, I did an internet search on this before posting, but I didn't find anything helpful, so I am posting here. My apologies if this has been posted before (I checked but didn't see anything).

While I was cleaning my tank, my Midas was trying unsuccessfully to hide under and behind a decoration that only his/her head could fit under. So I took the glorious opportunity to touch him. This is really the first time I've been able to touch him while he was confined enough not to be able to speed away. Upon lightly resting my fingers on his side, I could feel pretty fast vibrating, while he didn't move from his spot. Surprised, I did it a couple more times and was able to feel it once more, again only while he was confined to a spot. I also couldn't visually detect any vibration as I watched it.

I was just wondering if anyone else has ever noticed this, or knows what it means (besides the obvious "don't touch me"). I was wondering if the other fish could hear it and if they used this to threaten each other or something. I have always watched my fish together (similar sized texas' with a single midas) and tried to figure out how the other fish know when the slowly approaching Midas wants them to get out or when he doesn't care that they are in his territory. What do you guys think?

OscarRobinson;832303; said:

My Red O does it all the time. He flares his gills at his reflection then starts to vibrate. Then when his reflection doesn't back down he darts across the tank and hides in the corner. He's definitely a little yellow school bus rider.

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My texas does that too, it's very funny looking, except he doesn't go and hide! But anyway, I am not sure if I was clear when explaining it, but when I was feeling the vibrations the midas wasn't moving at all. It kind of reminds me of a chirping cricket, how you can feel their chirping vibrations.

Optimist describes it well but my fish didn't vibrate that strongly, so I guess I am not the only one who has felt it. Maybe if more people pet their fish...