I have an FX5.
Intake is about 4" from sand substrate. No sand issues.
However, during priming from my bucket, I accidentally got a tiny bit of sand in mine but no immediate problems.
Been running about the same amount of time as yours, maybe a couple weeks longer.
Noticed low flow, serviced the filters baskets, found excessive BB in my mechanical filter.
It was the cut-to-fit filter material and the water could no longer get by as the filter was cruddy with goop.
Flow returned to normal after I tore up the round filter.
No issue with noises, but did get the cavitation you mentioned.
Try playing with your filter materials a bit to see if that has any effect.
Is the flow rate as strong as when new?