HELP! Refrigerator knocks when toilets flushed

I’ve lived in this 13 year old home for one year. About 2 months ago, out of the blue, the refrigerator began making a very loud knocking noise each time the toilets are flushed. There are 3 toilets in the house….1 upstairs, 1 in master and one right next to kitchen. The one next to kitchen doesn’t always make the sound but the others do. A plumber checked and our pressure regulator to the house had gone bad (120 psi’s into house.) He replaced that today and the noise is slightly quieter but hasn’t gone away. Any ideas what could cause this? We can also see a pulse in the water (if a faucet is turned on) simultaneous with the knocking sound. There do not SEEM to be any other problems. The noise is obnoxious and I’m concerned there could be something wrong that will cause additional issues later if not fixed. The plumber who replaced the pressure regulator was otherwise baffled. Any help would be greatly appreciated!!