The pistons don't back off enough for the pads to rattle if the springs are missing. They just help prevent the pads from dragging and causing unnecessary heat.
The piston has enough range to burn through even the metal backing of a pad... you'd hear grinding long before it would start rattling.
Check for a broken or cracked pad or rotor. Check to make sure all the glides are firmly in place and that the caliper brackets and the calipers are all firmly connected.
Don't discount the possibility of a damaged suspension component either.
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