My imagination?

So, I have an 05 with 42k. Changed tranny and rear diff at 35. Have always used a synthetic blend oil up until the last change when I switched to Mobil 1 full syn. The Truck drives like a dream on the highway but when I'm in stop and go, I feel a slight vibration in the steering wheel and when I'm idle at a light, my engine has a whirl to it. Is a very slight whirl normal and I'm just hearing it now? I even fill up with Premium. Mechanic said everything looked fine when I went in for my 35000 check. Just said my plugs will need to be replaced soon. Thanks guys....