Heater is only 2 years old. Problem started last year. The heater is making like a popcorn sound (cracking). The flame is on and is heating the pool ,but sounds like the pipes are going to burst. I have no leaks and the water has been strong back to the pool until yesterday.I ran the filter and when I checked it the pressure was up and the water was very weak to the pool.I disconnected the heater and the water flow seems OK now.

You are not getting enough flow through the heater. Usually there is a flow adjusting valve somewhere on the unit or lines that led to the unit. In that amount of time I do not think it could be limed up but it is possible. Do you have hard water in the area?