Engine/Transmission (1998.5 - 2002) plug in heater vibration

[calexander]

Starting to get chilly here, plugged the truck in and started to hear a vibration noise coming from the truck. You can feel it in the hoses and actually see the water vibrate if you remove the radiator cap. Yes, the water level is fine, and it does keep the truck warm. Any ideas as what is causing the heater to vibrate the system?

 

[illflem]

The coolant right next to the heater element is actually boiling, it's normal. I can usually only hear mine for the first couple minutes after it's plugged in unless I pop the hood and listen closely.