3.5L LX9, Rough Idle

[vonVeezelsnider]

Since my car has been sitting amongst banged up cars in the dealership's body shop, I've been driving the 2006 "Rubbermaid Rattletrap" Malibu Maxx which my mom generally drives. I've noticed over the last week that the engine is acting kind of odd. When the engine is in idle (warm or cold engine) in any gear it's been tending to idle funny. You can feel the whole engine pulsing through the seat, the floorpan etc. Definitely not a serious miss, but certainly abnormal enough that you notice it right away. The engine doesn't sound too much different so from the outside it's almost imperceptible. One thing of note is that when the pulsing is happening the tach needle isn't jumping around, it's staying relatively constant.

The car has just over 38000mi/61000km on it and has the Hydramatic 4T45E Transmission. I ran some Techron through when I filled up earlier in the week and took it out on the two lanes out of town to get some of the carbon out but that doesn't seem to have done anything. Also of note that within the last 2000km it's had a fuel-air induction service at the dealer, but upon conferring the symptoms with my mom she said it has definitely been doing this on and off before that service was done, just more so recently

Any idea what I'm looking at here? It's not a strong enough miss to be really worried about but certainly more than enough to be annoying.

[Drew Dowdell]

Bad plug wire or dying coil pack

[Mike The Canadian]

It could also be an engine or tranny mount going bad.

Does it smooth out in park or neutral?