I have inground pool with a sand filter, from last couple days, after start the pump, the filter and heater vibrates and pressue gauge reading is fluctuating very fast. If I put filter to recirculate stage, the problem went away. If put to filter stage, it happened again. I already back washed filter, water level is good too. When vibrating, there is still water return to pool. any idea why this happens and what's solution to fix this?