RHF. thanks for all of the links. I'm doing mediumwave DX here in northern California, and I have found four frequencies so far that are surrounded by awfully loud noise that wasn't there before. This is disturbing as I was able to log stations on 730, 750, 900, 920, 1300, and 1320 back in 2006.

The frequencies (and likely noise providers) a

740 KCBS (now identifies on-air as "KCBS and KCBS-HD")
910 KNEW (mentions their HD broadcast on their website)
1050 KNBR/KTCT (no HD proof found yet)
1310 KMKY (no HD proof found yet)

I started an email exchange with the engineers at KCBS. Their HD broadcast is 24/7 so I have little hope of actually confirming that the HD broadcast is the problem, or of logging any stations on 730 or 750 ever again. Sigh.