Anyone have an opinion on this.

I have a Hayward H100 pool heater. We use it quite a bit. I've noticed that starting last week, when the water runs through it but the unit is not operating, it is quiet. However, once I turn on the heater and the unit fires up, after about half a minute it sounds like there's a rock or something banging around inside the unit. Turn off the gas, sound stops. Turn the heat back on, the clanking sound starts again.

Any ideas? Is this a dangerous situation? I have about 3 weeks left of the season here in NYC, once the kids head back to school pool usage essentially stops, so I can take the unit apart then and see what I can tell. But I'm just wondering if I should stop using it.

Any help is appreciated.

Thanks,
Phil