Interesting question
, we operate the B-757SF without the left pack in order to minimize the ambient sound , you can however turn off the right recirc fan to provide a more rapid exchange of air.May be the noise stops because Air moving from the cabin to the lower deck exhausts now exclusively thru the outflow vlv so air stops being suck by the R Recirc vlv to return to the mix manifold, as well of course the noise caused by the elec motor stops as well.We should ask an mechanic to be sure .
