It was an unmistakable sound in the most unexpected of locations.

"I've got two kids of my own, so I know what a baby screaming sounds like," Rydalmere resident David Otte said.

Found: the boy was down the drain for six days.

But the father of two said it was "unbelievable" to find a newborn boy down a western Sydney stormwater drain along a Quakers Hill bike path next to the M7 on Sunday.

The discovery sparked a police search for the boy's mother – identified later as a 30-year-old Quakers Hill woman – and calls for the public to withhold judgment until it became clear what had led to the baby being abandoned.