Of late I'm noticing rumble and vibration between 2500 and 2800 revs in 4th during light acceleration. Especially during warm up. It's also causing something to rattle around. I've heard others make mention of a low-end rumble with the 3.5L in the TL. Apparently this is why they add the noise cancelling system. But I haven't felt it until recently. Got about 16K miles on it. Any other V6 owners notice this?

 

could be a heat shield vibrating against exhaust components