pipes making noise all of a sudden

I have a water source heat pump in my house... I have been here about 10 years. This unit is a 5 ton and is 4 years old. Last night about 4:30am I awoke with a loud loud sound coming from my closet off of the bath. I went in there and it was the pipes under the floor banging on the floor. This right were they come into the house from the ground through the brick. This does not do this all of the time. I might do it 5 min and then stop.. I'm thinking it must be AIR POCKET maybe a pipe come a loose from the joist? I guess I better wrap up and go under there. The trap door opening is at the other end of the house were the Heat Pump is.. I got to sliver like a rat in the cold. all the way to the knocking end. . I have never had this happen.. My unit works fine.. The actual unit is on the opposite end of the house. I walked to that end but heard nothing. My house is 3,800 sq feet.. I have 2 ton heat pump up stairs.. I have not gone under there yet. I thought I would get usenets best to give me some Ideals If it is AIR is it coming in my underground tank???. Thanks David Tennessee
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