I noticed today that when I flush the toilet, fairly new low flow American Standard Champion 4, a roar follows for 30 seconds or so. It almost sounds like an elecrical motor. Even sound with no variation. I haven't had time to do a lot of research but I did notice that the sound was louder away from the toilet. It seemed to be the loudest about 6 feet away on an outside wall. Any ideas? Thanks so much.

Try closing the fill valve under the tank, then flush. Did you still hear the sound? If no it is likely the valve inside the tank which is easy and cheap to swap out. If you follow this route I'd recommend a Fluidmaster 400A