take them with you on the test drive. make the noise happen with them listening. if possible make a sound recording with your phone.
now that your rear brakes work correctly it may be making something thump. it is possible that the rear end has a loose part (suspension or even spare tire or something in trunk) that is thumping, but earlier wasn’t being effected by the brakes.
but… they have to hear it to fix it.
out of curiosity have you had a flat tire lately?