What did the folks at CCrane say about the results you could expect from hooking up to your roof antenna?
Background noise fluctuates on AM from day to day or night to night... a good bit of it is caused by sunspot activity... gamma rays, charged particles (ions), etc. Not much you can do about that (that I know of).
That's why I suggested checking the stations' websites to see if they offer streaming audio... that wouldn't be affected
AFAIK.