I have a Raypak 407A, its under warranty so I'll be putting a call in but I like to try and understand what is happening. The problem is about 5 seconds after firing up the burners there is a vibration at the inlet / outlet side of the heater. It's pretty loud, heater doesn't shake but it's not far off. It sounds like someone shaking a tin of ball bearings. This noise goes on for about 3 seconds then stops, after that all is fine. The heater runs and warms the water.
Any thoughts? It is a consistent problem, will happen every time you start the heater.