I think my daughter's house has a shuddering pressure reduction valve (PRV). Just as the toilets are about to completely fill and stop, her pipes begin to shudder. The shuddering gradually gets softer and then goes away about 15 seconds after either toilet has stopped filling. The noise also happens after her washing machine stops filling during wash cycles. So, I'm wondering if there's no harm in lowering the water pressure at the PRV a small bit at a time between toilet flushes to see if the valve responds better??
Other than the PRV, I can't think of what else it might be. It's definitely not pipes banging from water hammer. Appreciate your thoughts.