Filter pressure - another one - Cartrigde and Heater

[nomotor]

I have been perusing the problems and solutions but haven't found one that directly relates to mine. So here's the story. 5 year old plaster in ground pool with Hayward cartridge and Heater system. I clean the filters regularly and noticed about a week ago that the pressure was reading about 24 psi (normal is 15-17 directly after cleaning filters). I figured the filters needed cleaning since it's been about 3months and there's been a lot of "weather" here. After cleaning the filters and reassembling everything the filter pressure reading was still at 24-25 psi. Perplexed. I also noticed that for the first time I could "hear" the water running through the heater. The flow of water is this:
From Navigator in pool => Filter => heater => spa => waterfall back to pool. When I removed the inlet that enters the heater, the amount of water (with the pump on) is HIGH and the pressure drops to about 2psi. OK, well that makes sense. However, upon reconnecting the inlet the amount of water flowing out of the heater and utlimately out of the spa into the pool "seems" considerably less than normal. HERE'S THE QUESTION. Is there something that could be wrong inside the heater that is restricting the flow of water out of the heater to the spa? I read somewhere about waterhammering (trapped air) but don't know if that applies here. Any advice before i call Leslie's would be appreicated.
Thanks