The knock that ain't

My 99 always had a knocking sound at idle when fully warmed up. I was told it was pistons, a rod and or excessive rod end play.
I pretty much ignored the sound for three years. Then I hear a guy talking about the noisy evaporative emissions valve on the firewall. So I yank the wire connector, no knock, hook it up, knock returns.
The valve is pulsing and despite the rubber mount you can hear it inside, but not so much under the hood.
I took it off the mount and hung it by a tie wrap...

Yep. That's the evap purge valve solenoid.

They do that. Some have had success with a more noise isolating bushing mount.

I just let it tap away...

You have a pic of the valve?