Camry 2012+ :: Intermittent Buzz Or High Frequency Vibration Under Slight Acceleration

2014 Camry, 4 cyl. 65K ... Wife's car has developed a vibration that is going to be hard to get a dealership service group to troubleshoot due to randomness. I drove it for 30-45 mins today and have this description:

-Vibration, either described as a 'buzz' or a 'high frequency vibration'
-Happens in the 28-38mph range (different points)
-Under very slight acceleration. (hard acceleration efforts produce nothing)
-Can't swear to it, but feel like it is always in 'high' gear
-feel it in 'seat of pants', NOT through the steering wheel
-Is faint on sound, but you can hear something
-Is typically only a second or two
-Is very hard to 'create', but then happens unexpectedly.
-My wife first reported it to me, and I took my son on the test drive today and he could feel it as well when it hit, but couldn't pinpoint it as it comes and goes so quickly, but again, you feel it in the seat/butt more than anywhere else.

I've checked lug nuts to try to eliminate something easy and they are fine.

Since it is relatively slow speeds and under very slight acceleration, can there be a 'solenoid' that is creating a false shift effort, but just vibrates the trans instead? Without any info other than what I have stated above, I'm leaning toward transmission, but again, probably not enough info to go on....