loud shaking / vibration around thermostatic mixing valve pipes

Hi All, Exhausted all the resources online - so posting here as last resort. Kindly help. I have weil mclain tankless gas boiler. The issue is when i hear sudden shaking of pipes around thermostatic mixing valve (Honeywell AM-1 series) - the shaking is so loud that I can hear it across all the floors - it continues for a while then stops on its own. I couldn't tie this back to hot or cold water consumption as it even starts in middle of the night. It happens when flame is on or off. The shaking stops immideately when I shut off the cold water going towards boiler and mixing valve. So just want to know - what might be the issue. reading from boiler dial - Pressure - 20 psi - Water temp in boiler - 140 F - H2O - 70 feet