Boiler making a real strange wooshing vibrating noise after repair?

if its an old style tub with the old br stems then you might have a stem washer going bad,you also could have a bad supply valve at the hot water heater but if it was the supply valve it would do it all the time,even when you used the kitchen or bathroom sinks.the mixing valve could be bad inside the wall where your shower handles are.i would turn the water off and replace the stem/stems where the handles are connected.you paid for the work and its making noise so i would call them back