Well it never ceases to amaze me that whenever I am having a mechanical issue, someone else on Bike Forums has had the same problem.
I put a new cassette (SRAM 11-26) on the other day, and I am now getting annoying rumbling/vibration in the smallest three cogs (11, 12, 13). Seems ,according to attached two threads, others have had same problem.
New Cassette Vibration
Noise and vibration from new SRAM cassette
While both threads discuss possible solutions (tighten lock ring, play with B-tension screw, get new chain) neither comes to definitive conclusion. I have tried the lock ring and b-tension ideas, no luck. My chain is a bit worn -- .75% according to my Park CC3 -- so I will be changing that. But I was wondering if anyone out there had solved this issue definitively since the above posts first appeared.