Heavy vibration at idle.

Finally got the wife to go test drive a GT and all was going great until we had to sit at a long traffic light after getting off the interstate. It was idling smooth and quiet and then all of a sudden it started making a VERY VERY loud rumble and the vibration could be felt throughout the car. First we couldn't believe that it could be the car so we were looking around for someone near us possbily with a loud subwoofer. I put the car in idle and revved it but the rumble continued at the same frequency so we couldn't see how it could be the engine. The rumble stopped after we were going about 20-30 mph. We encountered another long light with the same scenerio but this time we new it was definitely this car. This was no boxer rumble. I'm thinking that maybe the cooling fan was kicking on and wasn't installed properly or imbalanced. What do you all think? Has anyone else experienced this rumble?

Chances are the dealer put cheap or low octane fuel in the car instead of what the car requires.

Wouldn't you think the frequency of the vibration would have increased along with rpms as it was revved in neutral though?

Yes it's the fan. I noticed this on my first test drive and it sounded pretty wierd just like you describe. Several of the cars I test-drove did it, noticeable when you are sitting at idle. I've only heard it a few times since I bought my Outback. So I would classify it as odd but probably normal. Defnitely something Subaru could improve upon. Craig

I think it's the fan too- when they're both on and you have the AC on and the windows up, it kind of hums.