We live in a 60 year old house that has 1' water pipes which are connected to our geothermal heating system. The geo was installed in the spring of 2010. The pipes run through the ceilings of the main floor and second floor. They are original.

I am hearing the sound as if there is a jackhammer being used on my street although it's quiet outside. I have used the special valve that is designed to relieve pressure but I'm unsure how much water to allow to drain out of the pipes. The sound comes on and off every few minutes but this is our first time experiencing this noise since we moved here 7 months ago.

How much water can I drain out before it's too much?

If this is a open loop system you need a slow opening and closing water valve to keep the water lines from hammering.