question about trickle filters...

I've recently 'upgraded' my tank to include a trickle filter design.
Basically, two pvc tubes take water in from just below the water level and
carry it down to the sump below. It's then pumped back up to the other side
of the tank. My question is, how do I make the input less noisy? I put fine
netting over top, and that reduced the sound somewhat, but now there's a
higher pitched occasional sucking sound. Any suggestions?
TIA

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