To see if it is bubbles or your sand turn off your pump. Do the things go up or down? Up they are bubbles, down the are sand. Check the inlet of your pump, is there a small whirlpool near it. If so that could be the bubbles. Another, though harder to find thing is a leak in your inlet plumbing. You think it would leak water, but it would actually suck air in the line and cause bubbles.