3.8 automatic (of course). Under light acceleration or cruise it seems to have a slight harmonic shake or vibration(slight shake, goes away for a couple seconds, slight shake, repeats), if I accelerate slightly more or let off the gas, it'll go away. No issue with OD or going into lockup, actually seems to occur before lockup. Under heavier acceleration it doesn't occur.

Changed filter and fluid and it's a little more prevalent that before.

Is there an additive that might stop this? It's not severe at all, wife (who's primary driver) doesn't even acknowledge it. And it's not horribly bad, just noticeable to this car guy.

Thanks