Rattling/Grinding Sound When Reaching Peak Acceleration

My ride: 2004 2wd 2RZ base cab, 268,000 original owner miles.

OK so this just started. It doesn't happen so much in first gear, mostly second and third. During acceleration- just "normal", nothing near "flooring it", seconds before it's time to up-shift, I get this rattling (grinding? rattling grind?) sound coming from under the tunnel area. If I let off on the gas, it will fade/stop. It does not do it in neutral. It's not that spherical plastic seat that the shift lever ball rides in (I forget the correct name). The sound seems to be coming from deeper and further forward than that.

Sound familiar?

 

It might be the cat converter rattling.

 

my thoughts also. something exhaust related.