Here in Torrance, powerhouse 640AM may be too powerful. I'm getting the same surges of slop background noise on my CC Radio Plus, my CCRadio-SW, my Sangean LB-100, and my Grundig S350. It appears to be a location problem. Rotating the radios sometimes helps; sometimes it doesn't. Today I was able to get rid of the horrible background noise (a distant police whistle or boat horn) by rotating the Grundig S350.
Two radios that don't seem to have the problem: Sangean WR-2 and Sangean WR-5.