Listen to AM all the time and have never noticed any RF interference from the HSD system.
But it could be S/N ratio isssue if the station signal is weak. AM 1150 has a pretty strong signal in my area. Does you RF interference happen at all bands or just the one you listen to most? Is your station a relatively stong signal?
Guess the stations are on the weak side, although, I also have a 2003 Avalon which gets the stations in a hell of a lot stronger and NO interference. Let's see, last time I looked I think I paid a lot more for the Lexus and this should not be an issue. I'm in the Poconos, and the closest and the strongest station is approx 45 miles, that's close as far as I'm concerned.